Episode: 1240
Title: HPR1240: Doomsday Rule
Source: https://hub.hackerpublicradio.org/ccdn.php?filename=/eps/hpr1240/hpr1240.mp3
Transcribed: 2025-10-17 22:10:27

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Hello and welcome to Hacker Public Radio.
I'm Charles in New Jersey and I'm back with a follow-up episode to my recreational math
introduction called The End Days of Christmas.
You can find those episodes at their episodes number 1143 and 1189 and be sure to get the
revised edited version of the second one.
In any case, I made an illusion in my very first episode to something called the Doomsday
Algorithm.
That algorithm is how I attack the problem of figuring out the day of the week for any
given date on the Gregorian calendar.
I can use this algorithm and find what day of the week some day in history is or even a
date that's in a few months or some years down the road.
I did not create this algorithm.
I live in New Jersey not far from Princeton University which is where John Conway is a professor.
John Conway is a man who's done some very cool math stuff and you should especially check
out his work on cellular automata and the game of life.
Now leave it at that because those are episodes in themselves just talking about them and
creating some Python programs that demonstrate how those things work.
But that's another episode for another day.
The Doomsday Algorithm helps you solve the problem of these pesky days of the week that
seem to move around each year.
Well, they don't move randomly exactly.
A non-leap year has 365 days.
If I want to find out what day of the week this day next year is going to be and this
is not a leap year when I'm recording this, I just add 365 days where today is the cocktail
party and the date that I want to find is 365 days down the road.
Well, if I want to date that's seven days down the road, I can count and find out that
if today is Tuesday, a date that's seven days later will also be Tuesday and I invite you
to count on your fingers to verify that if you like.
But I think if you just look at a paper calendar that's hanging on a wall or a calendar that's
in a month view on a computer screen, you'll see that the Tuesdays and the month are
seven days apart.
If the second of the month falls on a Tuesday, then the second and the ninth and the
16th and the 23rd and the 30th will all be on a Tuesday.
So for every time I have a seven day stretch, if I subtract that from my number of days
that I'm counting out there, it really doesn't matter because I've just eliminated one time
around the cycle of one week.
So 365 days gives me the same offset from today's day of the week that 15 days does because
I've just subtracted 50 weeks right off the top.
I've just gone around the week cycle 50 times without changing the date by eliminating
that 350 days.
Well 15 days is just a fortnight and an extra day or for Americans it's just two weeks
and an extra day.
So the day of the week for a date that's 365 days in the future shifts the date forward
by one day.
Sadly you've already noticed that your birthday moves about one day forward in the week each
year that passes except for leap years where you have an extra day and if your birthday
is in January or February, it jumps two days on the year after a leap year.
If you're in the latter 10 months of the year, your birthday moves forward two days in
the leap year itself.
That leap day on February 29th that actually pushes everything forward that extra day should
be straightforward.
Okay.
Well what that does is it creates a lot of jogging around of the day so you can't just
say well in seven years my birthday will fall on a Wednesday again.
You can't be sure because it depends on how many leap years are in between.
If next year was a leap year then you'd only have to wait five years for your birthday
to go around the cycle of one week and land on the same day.
If your next leap year is two years out you would have to wait an extra long time because
what would happen is the first year would advance it one day then the leap year takes
it forward two more days that's three in two years then the third, fourth and fifth
years move it out to the fourth, fifth and sixth day respectively.
Now that sixth year is a leap year.
So now when that leap year comes you jump over your birthday instead of having it fall
on Wednesday again it's now Thursday because you've skipped over it and then let's see
just another five years before it actually gets back to where it was.
So you're now talking about 11 years before your birthday comes back on the actual day
of the week.
Now it's gone by it once but it has actually taken 11 years to rotate around two exactly
falling on that date.
So it can be a little tricky to figure out when a certain date on the calendar is going
to rotate back around to be on a given day of the week.
So that's why it's nice to be able to solve the other problem which is given a date in
history or a date in the future.
How can I tell what day of the week that will be or what day of the week it was.
So let's take a date that's significant to a lot of baby boomers.
I used to hear people asking wherever you when JFK was assassinated what were you doing
when you heard.
Well that date was 22 November 1963.
What day of the week was that?
Well the Doomsday Algorithm gives me a couple of touchstones.
It gives me a collection of days of the year that are both memorable and that all fall
on the same day of the week.
So if I can identify when any one of them falls on a certain day of the week then all of
those dates in the calendar will fall on that same day.
Sick wise I get a day of the week that's kind of an anchor for these sets of fixed
reference dates that we call Doomsday as of the first year of that actually of the last
year of the prior century.
So in particular if I have the year 2000 itself this Doomsday set of dates falls on a Tuesday.
In 1900 it was Wednesday in 1800 it was Friday.
In 1900 it was Sunday, 1600 it was Tuesday again.
And I'll explain how I got to that a little bit later.
So we're looking for 22 November 1963.
Well since I'm in 1900 my anchor day of the week is Wednesday and because I'm in 1963
I'll do this in the brute force method.
That's 63 years into the century which is 9 times 7 so I can say okay that's put me
back in the place where I started and then I've got to count the 15 leap years that fell
in 1904, 1908 up to 1960 and 60 divided by 4 is 15.
So I've got 63 days that the year years have passed and each one contributed one day shift
and then 15 days in there, 15 years in there each contributed an extra one day shift.
So I need to add the two.
Well 63 basically is the same as zero and 15 is essentially the same as one because it's
a fortnight plus one day so 1963 is basically only one day ahead of 1900 in terms of the
day of the week where every one of those dates falls.
Okay.
One of the anchor dates for November is November 7 so that was a Thursday.
Because I'm one day ahead of 1900 and 1900 doomsdays are on Wednesday then my doomsday
for 1963 is Thursday and November 7 is one of those which means that November 21 is
also one of those.
Since November 21 was a Thursday, 22 November 1963 was a Friday which is actually the
same day of the week that it is this year, 50 years later.
And before you start making that into a theorem, it doesn't work for 22 November 1964.
Because that date is late in the year, 22 November 1964 jumped from Friday to Sunday whereas
22 November 2014 is only going to advance by one day because this is not a leap here
and neither is next year.
So next year November 22nd will be on a Saturday.
So that 50 year rule is not going to work in this case.
Okay.
Wow.
How did you do that?
Well, I think before we get too far into this we should probably take a little trip into
a subset of calendar math that I'm going to call week math or math that you do on the weekly
cycle.
I'll start very small and with a fixed date.
I'm going to choose Tuesday June 6th year 2000.
Now if I want to go three days ahead of that, that would be June 9th.
The 6 plus 3 is 9, that's the easy part.
And now I'm going to get some practice counting out the days.
June 6th is a Tuesday.
So three days after that would be one is Wednesday, two is Thursday, three is Friday.
So June 9th year 2000 was a Friday.
Just to keep myself in practice, what was three days before that?
Well, that would be June 3rd.
And once again you can use your fingers.
June 6th was Tuesday, the 5th was Monday, the 4th was Sunday, the 3rd would be Saturday.
Okay.
Now suppose I go four days ahead of June 6th 2000.
Well, since June 9th was on Friday, we just did that.
Four days after would be the 10th and it would be the next day so that would be Saturday.
If you'll notice I've computed Saturday June 10th is four days after.
Saturday June 3rd is three days before.
So if I'm trying to figure out the spread between those days, I can add those numbers
because I'm at the cocktail party for both of those counts.
And four plus three is seven and they should be on the same date.
We may use that later.
So it doesn't really matter if you're counting backwards or forward.
If you've gone a multiple of seven in all, then you've done a full cycle around the week
and you're not going to affect the day of the week relative to your starting point.
That's going to be important.
So now let's suppose I want to go 29 days forward from June 6th, year 2000, which is
the Tuesday.
Well, what day of the week should that be?
Well, since 28 plus one is 29 and 28 is a multiple of seven, all I've done is spend
my wheel four times and I'm back on the same day of the week.
So I'm really, if I'm adding 29 in terms of days of the week, I'm just adding one.
I'm adding a remainder or an offset of one.
If you think of it that way, you can whittle down problems to a much easier set of things
to solve.
So what day of the week is 6 June plus 29 days?
Well, 6 June was a Tuesday.
So that date must be a Wednesday.
Now to check it, 6 June 2000 plus 29 would be effectively June 35, remember we learned
how to do this last time.
And because June has 30 days, that would be 5 July and since 11 July is one of my Doomsday
reference dates, which I will list for you soon, then 4 July is also and 4 July is Tuesday
as is June 6.
So July 5th is again on Wednesday.
So everything's working.
Okay.
Now, we can start deriving the Doomsday rule and using it.
Now there's some basic insights we can have here and we'll start with Doomsday itself.
I think you should think of that as the last day of February because that's the date
that's really moving things around in the calendar when you have a leap year.
So we might as well just focus on it because that's the one that's going to be our pivot,
whether it's the 28th of February or whether it's the 29th of February on a leap year.
So we might as well use that as a starting point.
Well, since we're using the last day of February as our anchor point, then you could either
say that March 0 is, which is the same thing, is also a Doomsday or it's probably more
convenient to say March 7 and you'll see why in a second because it's just one week later
it's an actual day of the month rather than some kind of artificial construct and it
turns out to be easier because it's consistent with a rule that's going to emerge in just
a few moments.
Now the months of the year, once you hit the month of April, the months of the year can
be arranged in pairs in which one month is 30 days and one month is 31 days.
Now sometimes those are in different order so that you get two months in a row of 31
days but they're bracketed by months that are 30 days long.
So let's find an anchor date for April.
Well since March 7 is the same day of the week as the last day of February, then March
35 would also be because it's 28 days later or four weeks and that translates to April
4.
Hmm, April 4, 4 4 is pretty easy to remember.
Well, because 4 4 is an anchor date, that's going to turn out to be really convenient
because since April has 30 days and May has 31 days, that's 61 days and that's just
two days short of nine weeks.
So to go from 4 4 to 4 June, gotcha, you thought I was going to do it in one or the other
of the number only format.
So what if I went from 4 4 6 6, that's nine weeks, that's the same day of the week.
And watch me now, June is also 30 days and July is 31.
So if I go from 6 6 to 8 8, same day of the week, it's 63 days, same day of the week.
So 4 4 6 6 8 8, but wait this more August has 31 days and September has 30.
So from 8 8 to 10 10, nine weeks, same day of the week, oh but October has 31 days, November
has 30, 10 10 to 12 12, 63 days, nine weeks, same day of the week.
So what I have so far, I have the last day of February and I have 4 4 6 6 8 8, 10 10 and
12 12, all the even numbers after that first couple of months, all in the same date, 4 4 6 6
8 8, 10 10 12 12, couldn't be easier.
Now the odd months are not that easy and what happens is in the early part of the year,
I take the number of the month and you add 4, so March 7, as we've already seen, I'm
going to claim that May 9, 7 11 are also, or May 9 and July 11, sorry, I got that backwards
for you, are also doomsdays.
And fortunately, I said 5 9 and 7 11, American style, well if you look at 5 9 and 11 7
or if he and, or international style dates, those are also doomsdays.
So 9 May, 5 September, both doomsdays, there's a sentence that people use as a mnemonic
for this that I work my 9 to 5 job at 7 11, which may work in the states because there's
a chain of convenience stores called 7 11, so we could remember that, okay, I'm working
a 9 to 5 shift at a store called 7 11, but however you remember it, in the first half
of the year it's the odd month you add 4, so March 7, May 9, July 11, and then you just
reverse those for the second part of the year, which would be 5 September and 7 November.
Now January and February are affected by the leap year, on a non-leap year, the last
day of February, which is the definition of doomsday, is the 28th, which means that January
31st, which is 28 days before that, if the cocktail party is on the 31st, then the 28th day
on is the last day of February.
So on a non-leap year, January 31st is a doomsday, which means it might be easier to think of 10
January, as the doomsday for January, and on a leap year, instead of saying February
1st and then having to back into it, you could say that 11 January is the doomsday reference
point for January, and February 29th, of course, would be the reference point for February.
However, you remember it is fine, just, you know, you can compute your favorite thing
to remember, there are a lot of possibilities, I don't want to limit you by forcing them
upon you.
Now it's nice to have these reference dates that I can use for any year that I'm in,
especially since they never move for the dates that are after February 29th, doomsday,
or 28th, if it's not there, and there's just a rule that I have to remember that it moves
a little bit in January for years that are leap year, instead of being 10 January,
it's 11 January, so you just need a friend whose birthday is 11 January, or get married
on that date, and then you can either always remember it, or I guess never remember it,
depending on what kind of person you are.
If you're male, you probably will forget it.
The next maneuver that I did that seemed a little bit iffy was this anchor date for
the century thing, and it turns out that you can figure out the way the dates move over
large numbers of years, quite easily, because there's this extra rule that you get with
leap years in that, yes, every fourth year is a leap year, unless it's a multiple of 100,
then it's not a leap year, well, unless it's a multiple of 400, then it is a leap year.
What that little exception does is it knocks you into a 400-year cycle, where every year
that is a multiple of 400, like, say, 1600, 2000, 2400, 2800, those are all years in which
all of the dates are aligned from 1 January to 31 December, so you've got these 400-year
cycles.
Now, how am I supposed to believe that I have 400-year cycles?
Well, let's count, okay?
Let's say I'll start at 1600 as my base year, and I want to see what happens to Doomsday
in 1700.
Well, I've got 100 years between 1600 and 1700, 1600 is the equivalent of the cocktail
party, so the 100 years are 1601, 1602, 1603, and so on up to 1700.
Each year is pushing the dates ahead by one, and then there are 25 of those years that are
multiples of four, one of which is not a leap year, so that's 24, so you only get 24
extra days out of that, so I could say that's a total of 100 plus 24 or 124, and what's
that in terms of weeks?
Well, here's a little trick.
The 100 is already a reduction of the 365 times 100 plus 24, so we're really reducing
36,524 to a remainder after you divide by seven, but as I said before, since each year
pushes the data head by one, I've already done this reduction of the 365 to one, and
I could just as well reduce the 100 to, well, that's 98 plus two, and if I just eliminate
the 98 because that's an even number of weeks, I end up with plus two.
Now the 24 leap days that I've seen in that century is actually 21 leap days, so I've
gone three whole weeks plus three extra days, so I've got two days from the plus one for
every year, and basically a net of plus three for the leap days, two and three is five,
you can think of it as pushing me ahead five days, or actually falling short of going around
the cycle completely by two days.
I prefer the minus two because it's a little easier, but you can do what you like, so what
happens is in the year 1600 itself, Doomstay is on a Tuesday, because it always is.
In 1700, I go forward a hundred years, and I'm two days short of going around the cycle
even number of weeks, so that Tuesday becomes an anchor date of Sunday for that century.
So everything I do within that 100 year period is going to be based off Sunday instead
of Tuesday.
I can do the same thing from 1700 to 1800, so the anchor date for 1800 and the 1800s
would be Sunday minus two days, or Friday, and the same thing applies from 1800 to 1900
and drive lost two days, so the anchor date for the 1900s would be Wednesday, two days
before Friday.
Now from 1900 to 2000, I have the same situation as I had before where I've lost two days
for everything up to 2000, but 2000 is a leap year, so I've actually only lost one date
in that century, so I'm back to Tuesday.
So I think we've just established that in every 400 year period that starts with a multiple
of 400 as the cocktail party year, then the days of the week cycle around completely
over that 400 year period, so that 400 year, year 2000, the end date, which is the cocktail
party for the next cycle, is the same set of days of the year as the beginning in 1600.
2400 would also have the D days, so it will all be on Tuesday, which would be January 11,
March 7, April 4, May 9, June 6, July 11, August 8, November 5, October 10, and December 12.
Those dates will all be on Tuesday in the year 2400.
I won't be around to see that, I'm not sure I want to be, because I'd be quite old
by that point.
Okay, now what happens within a century?
Well, because I need an offset of plus one for every year that's transpired, I know I've
got that as my base, so if I've got a year within the century, I know I have to add
at least that many days, and then I have to add the number of leap years that have already
occurred, including if the current year is a leap year.
Because D day is the last day of February, I'm not missing anything by including the current
year as a leap year, because my doomsday date adjusts to the last day of February, whatever
that is.
So good.
So now let's do a couple of examples, because that'll help us fix ideas.
And I guess one example we could try, that would be easy to remember, would be D day,
the date of the Normandy invasion, which would be 6 June 1944, what kind of day of the week
was that?
Well, I'm going to do the brute force version before I introduce the shortcut.
And that is, I take the 1900 anchor, and that's Wednesday, and if you want to figure
that out, it's 1600 is the greatest multiple of 400 that's less than the date, and I just
subtract off two days from Tuesday for each hundred years.
So Tuesday, Sunday, Friday, Wednesday.
Okay.
So Wednesday is the base anchor date for the 1900s.
Now I'm 44 years into that century, and I have 11 leap years, including 1944 itself.
So I can either say that that's plus 55, and notice that's one less than 56, and say
it's minus one, or I could have said that 44 is 42 plus two, and 11 is seven plus four,
and so that two plus four is six, which is also minus one.
So the Doomsday would be on Tuesday for 1944, but as we've already seen, June 6 is one
of the monthly anchor dates, so D-Day was Tuesday.
For a second example, one of my relatives was born on January 20, 1898.
Okay, now the 1800 anchor is Friday, so for that whole century, I'm anchoring on Friday.
Now I've got a 98-year offset, and 98 happens to be a multiple of seven, so I can just neglect
that entirely.
And I had the maximum number of leap years, which is 24, and that is 21 plus three.
So the Doomsday would be Friday plus three, which is Saturday plus two, Sunday plus one,
or Monday.
So that means the Doomsday dates are all on Monday that year.
So since 1898 is not a leap year, we have the 28th of February is Monday, which means
the 31st of January is Monday.
And two weeks before that, so I'm adding, is the 17th of January, and that's also Monday.
Since the 20th is three days later, the 20th of January, 1898 is Thursday.
Now just to show you that this is not pure trickery, I could do this by the extreme brute force
method.
That is the anchor date for 1600, the Doomsday date in 1600 is Tuesday.
1898 is 298 years after 1600, of 280 of those years, just take me around the week about
40 times, leaving remainder 18, which is four more than 14, which is two weeks.
So the 298 one day shifts give me a net of plus four.
Now in that 298 years, there were 74 multiples of four at 1604, blah, blah, blah, blah,
1896.
And there were 74 of those, but two of them were century ending dates, which were not leap
years.
So we can't count them in the leap year offset count.
So actually my leap year offset is now 72, which is two more than 70, and I've got four
and two is six.
So for that year, the Doomsday is on not Tuesday, but Monday.
That's using an almost 300 year offset, and I still get the same Doomsday result.
Wow.
Factoring 98 and finding number weeks in there and finding the leap years is dealing with
numbers that are getting a little bit challenging for most people's ability to do mental arithmetic.
So there must be a way to make this a little simpler to use smaller numbers, and I think
we can.
And the way Conway proposed was to look at the sequence of 12 year cycles that you can
fit into a century.
And there's only eight of those and a little bit over.
So you're not really talking about a lot to keep track of.
Why would you choose the number 12?
Well, if you look at the first 12 years of a century, let's use the 1900s as an example
because that's fairly easy to look at.
Now 1900s the cocktail party.
So the first 12 years are actually 1901, 1902 up to 1912.
Since each year pushes the day of the week for each date forward by plus one, I've got
a plus 12 offset right there.
And in those 12 years, I've got how many leap years I have three.
I can even name them.
They're in 1904, 1908, 1912.
And 1912 becomes the cocktail party for the next set of 12.
So we get the same behavior.
Because I have plus 12 from the years themselves and additional three from the leap years in
that 12 year cycle, I have a total offset of plus 15, which gives me a plus one for the
entire 12 years.
And that's all the information I really need.
I don't care how many times I went through the cycle.
I don't care that it's actually 15 days.
The plus one is enough information.
So if I wanted zero in on, let's say, D day in 1944, the same date we did before, another
way to look at that is how many of these 12 year cycles have already passed, how far
I am into the current one, and how many leap years have I seen in the current 12 year
cycle?
Well, 44 is three times 12, which is 36, plus eight years into the current cycle.
And then how many leap years have I had in the current cycle, that's the eight years
divided by four.
And remember, if this year is a leap year, I count that.
So the offset for 1944 is going to be three from the three times 12, plus eight, which
is really plus one net.
So three in one is four, and then eight over four, that's the number of leap years,
but it's actually equal to two.
So I've got three plus one plus two, it's minus one that's one down from the anchor date
of the century, which is Wednesday.
So D day is again on Tuesday, and six June is one of the one of the doomsday reference
dates for the year.
So very cool.
Now for the 1898 thing, well, instead of having 98 is how many sevens and so on, well 98
is eight times 12 plus two, and I'm two years into this, so I haven't had a leap year.
So my offset is eight plus two plus zero, because I have no leap years, and that's ten,
which is equivalent to plus three, because I don't care about that extra week.
So I'm plus three, and Friday plus three is Monday, and I'm back where I was in 1898.
And that means January 17 was a Monday, and I believe that's what I said in my original
comments.
So we have a way to figure out what our date of birth was, or our wedding anniversary,
days of the week, and so on.
How would this be useful, let's say, in planning a meeting?
You never know, there's always something that needs to be planned a few weeks in advance.
So let's say I'm going somewhere September 28th.
What day of the week is that this year?
As I'm recording is 2013, doomsdays are on Thursday this year.
Now in September, 9.5 if you're in the states, 5.9 if you're anywhere else would fall on
a Thursday, which means that the 12th, the 19th, the 26th is Thursday, and the 28th is
two days after that.
Let's see, I can use my fingers now, I'm down to two days.
The 26th is Thursday, 27th is Friday, 28th is Saturday.
See, it's handy even within a year, you don't have to do a lot of calculating, especially
if you are aware of this to the point where you know the doomsday for this year, you don't
have to calculate it again and again.
For 2013, Thursday, right?
The 2014 is going to be Friday, 2015, it's going to be Saturday, 2016, now it's going
to jump and it will be, what, it will be Monday.
Now maybe I should check that for the year 2000, anchor date is Tuesday, 2016, okay,
16 is 1 times 12 plus 4, so I got plus 1 for having gone through an entire 12 year cycle.
I've got 4 years into the current cycle and it's a leap year so I count that one too.
So 1 and 4 and 1 is 6, which is minus 1, so doomsday is Monday, which is what I hope I
just said.
That's really all you need to know to both explain how it works, this doomsday rule,
and to use it in practice.
Now if you want to do one of the brute force calculations, that might be okay for a spreadsheet
where you have tools that will help you like the mod function.
All that stuff I was doing by reducing the numbers by throwing out all the extra weeks,
a spreadsheet like numeric or library office, calc or the dreaded Excel or whatever, is
going to have a mod function and what that does is it returns that offset number that
I've been seeking all this time, which is the remainder when you divide by 7.
It strikes out all of the whole number times that 7 divides the number that you're trying
to evaluate and gives you just the remainder.
That's how the mod function works.
To confirm that offset in a century that does not end in a multiple of 400, I can count
the number of days in that century, which would be 36,524, then if I just plug that number
into a formula by saying mod then the number 36,524, comma 7, that will give you the remainder
from dividing 36,524 by 7 and it should come out to be the number 5.
In any case, that's the spreadsheet trick that would let you use ultra brute force methods
to do this calculation.
I think what I will do is have a spreadsheet that will have one sheet that does the
doomsday rule with the division by 12, the remainder and the number of leap years in
the current cycle and figures out the offsets and the centuries and even the doomsday numbers
if I do a table look up on the doomsday numbers then you'll have them and you won't forget
them.
The evens are easy to remember because it's 4, 4, 6, 6, 8, 8, 10, 10, 12, 12.
The odds are 3 plus 4 is 7, so 3, 7, 5, 9, 7, 11 and then you can flip those to get 9,
5 and 11, 7, whether you're living in the US or Europe, those should all be fine.
If you're not in the US then you want to remember 7 March, so it'd be 7-3 as opposed
to 3-7 and then it's the last day of February, whether it's the 28th or the 29th and in a non-leap
year, the January reference date is the 10th in the leap year it's the 11th.
That pretty much sums that up so when I give you the spreadsheet and I do a look up on
that first very detailed page, it'll have a look up table where you can refresh your
memory and look at those again.
Or you can modify them to use dates that mean more to you.
If your aunt Frida has a birthday in January that happens to be one of the target dates
then maybe you'll remember it better by having aunt Frida's birthday as opposed to one
of the anchor dates that I proposed.
Okay and then I'll do the more brute force approach where I just did the divisions and
the mod 7 calculations within a century and stretching all the way back to the base
of the century, the 1600 for that 298 year calculation.
It's all the same calculation really, it's just that these extra maneuvers are to make
it a little bit more convenient for humans who are doing mental math.
The creator of this rule, John Conway, has a program on his computer that generates
a date so it gives him a date he has to calculate the day of the week on which that date falls
and his program only gives him a few seconds and he can do it in that short a time because
he's been practicing it for years so the doomsday portion of his brain is pretty well developed.
But I'll probably rot your brains by giving you the spreadsheet that gives you ways to
get at the answer and lets you play with how you do it so you can actually come up with
your own variant.
If you look on Wikipedia, the article on doomsday rule, there and that will be a link
in the show notes as well.
There's actually a faster way to get at this but it's got a quantity that I'm not sure
everyone who is listening to this will remember.
But if you want to look that up, program it, use it, I'm sure it's fine.
I don't find that it takes me a long time to do this calculation.
In fact, the thing that takes me the longest is figuring out the day of the week for a
date in the month that's not on the same day as doomsday.
So it's that last one or two or three day, maybe four day offset that.
It's usually the last couple of steps that throw me off and not the big multiplications
or divisions that are causing any problems.
So that's all I have for today.
You can practice up and amaze your friends with your newfound skills in computing the
days of the week of any date and you'll be able to use this for fun and profit.
This is an interesting topic.
It could be helpful to you.
I hope you've enjoyed this episode and that you'll be able to use this for fun and
profit.
So until next time, I'll be looking forward to your episode on Hacker Public Radio.
Think about putting it in a show, okay, thanks.
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