# How many calendars are there?

How many calendars are there? It turns out just one and it's calculators of
1974! I mean come on if you had a calendar with this in it why on earth
would you ever need any other calendar? I mean look at that. Awh.
In that case I'm gonna look specifically at how many calendars there
are in the Gregorian calendar date keeping system. And of course throughout
history different countries and cultures have had their own calendars and that
continues to this very day. If you get a phone and you look at the calendar
setting... I'm now recording this at the very end of 2018. 2019 comes next. If I go
over into my general settings, not date and time, but under that languages and
region I can change the calendar. So let's go Buddhist and now all of a
sudden if we come back out to the calendar it's the Year 2561.
Maybe borrow a friend's phone change the calendar and then try to convince them
that just woken up from a coma. Even though there are other calendars the
Gregorian calendar is the one used by most countries and it's the one that the
official ISO date keeping standards are based upon. So I'm gonna look at the
Gregorian calendar and see how many of those there are. 14. There are, I mean
that's the answer, there 14 different options. Probably should've started with that.
Sorry. Wow classic YouTube we got there eventually. Could've just started watching here
it turns out. So there are only seven different days that the year can start
with and it's either a leap year or it's not that's it.
14 options every single year is one of those so 2019 starts on a Tuesday not a
leap year. 2020 starts on a Wednesday: is a leap year. 14. Because 2019 starts
on a Tuesday there's not a leap year you can use any calendar from any other year
which started on a Tuesday and wasn't a leap year. Including 1985. Which is why I went and
bought the culture club calendar from 1985. And this, this was.. ha, that is peak 80s
right there. And this... and this is...
That is not even a joke. Right, this is not like someone making fun of the 80s later
this was made in 1984 this was done deadly seriously. And in the interest of
balance I didn't just buy the culture club I also got the scorpions. Here we are.
More 1980s but in a very different fashion. That's pretty... ok that's pretty 80s
oh that's very 80s aha look at that right so these start on a Tuesday not
leap years and so I can use them actually bought these in 2012 because
2013 started on a Tuesday it wasn't a leap year so these were my calendars from
2013 and now I can reuse them in 2019 and likewise 2020 find any calendar
which started on a Wednesday and was leap year use it again. Confession time
the calculators in 1974 I made this myself because I thought I was hilarious
This is my own calculator calendar made for my own personal use but 1974 also
started on a Tuesday and wasn't a leap year so I thought it'd be very funny to
make a calendar which is based on 1974 but works in 2019 and the other problem
with 2020 is because it's a leap year those calendars are bit more rare they
don't come up as often. So you need to go back to 1992 if you want to get yourself
a 2020 calendar. I mean that's pretty rad and then you can save it until you get
to 2048. The next time it would be used. Because 2048 is as far from 2020 as 1992
is. Not rad. And when I was making my calculators of 1974 calendar I did the
actual like month grid myself I generated these in Excel and I couldn't
be bothered changing the length depending on the month I thought it'd be
funnier to have a set 7 by 5 grid and then I put a 1 on whatever day in the
week the month happens to start on it then it just fills in the rest of it so
you can see here March will start on a Friday here that starts on a Monday right
but then here I propagated all the way through to 35 you've gotta know when to turn
over to the next page and a lot of them if they start later in the week.. where's
one that's later in the week... here we go this one doesn't start till Saturday and
so I put zero and negative so you can actually turn the page over early if you
want start using the negative numbers of the next month. And it made me realise
that every single month there's only seven options for each month apart from
February so February can have two different lengths and they can have
seven different starting dates so it gets fourteen again but otherwise every
single month only has seven options for that specific month across all months
seven starting points there are four different lengths of a month there are
only 28 different possible months which makes things like this make me very
angry. Good luck everyone this year December has five Mondays five
Saturdays and five Sundays this happens once every 823 yea.. No it doesn't! These
things online really annoy me I I get the argument they're just some harmless
fun. But honestly, get a calendar and check. You can look up the month where this
happens. All months that have more than 28 days will have five occurrences of a
particular day within them and all the months that are 31 days long like
December is always gonna have three days I don't why they change the order here
but so Saturday Sunday Monday... if it starts on the Saturday you're gonna have
a bonus Saturday Sunday Monday on the end that's going to happen every single time
But yet this stuff gets passed around and actually if you want some good
ammunition for the next time you're annoyed by something like this on the
internet we can have a look at how long it takes a Gregorian calendar to loop
completely. If you take my list from before of the types of year and set it
going eventually it will loop, it will perfectly repeat itself. I actually made
this list in Excel putting in the formula to work out leap years is kind
of fun because if the year is multiple of four it is a leap year unless it's a
multiple of 100 then it's not unless it's a multiple of 400 and then it is
again and that means that the leap year system in the Gregorian calendar repeats
every 400 years and because 400 years worth of days: it's 400 times 365 plus
the extra 97 leap days, that total is a multiple of seven: the day's repeat at
exactly the same time that the leap years do and so every 400 years the Gregorian
calendar perfectly loops back on itself. So the day you're enjoying now is the
same as the day 400 years ago and 400 years from now
2019 is a Tuesday non-leap year as was 1619 as will 2419. Although I appreciate there are some complications because
like in different countries they switch calendars at different times so if you
go back into different calendar system so in England in 1752 they switch
calendar systems took out 13 days but didn't change the order of the day so
that breaks that going backwards and who knows what's going to change going
forwards but for now on this calendar system nothing can happen less
frequently than every 400 days that's the longest you can go for any weird
combination of days and dates that you want it'll repeat at worst every 400
days it cannot, cannot be every 823 years. I have no idea where that number
originally came from if anyone can find out and tell me where 823 years comes
from I would love to hear it. Although none of this factors in cultural or
religious or personal days and some people are thinking:well hang-on! Generic
religious holiday that moves around that doesn't loop every 400 years. You can't
say calendars repeat ever 400 years, those dates don't! And the thing is that
changes for everyone. So when I made calculators of 1974 I thought you know
what I'm gonna fill in all the days that are important to me. And so on the 31st
of December 18 which is the zeroth I turn twenty million minutes old so that
is my 2 times 10 to the 7 minute birthday. Straight after Christmas which
you'll note I haven't bothered putting in. Straight after that the 26th
that is my 2,000 day wedding anniversary and that's probably
worth noting because my wife and I celebrate every multiple of five hundred
days because that makes more sense and I better not forget that, that's a very
important day that myself and Lucie celebrate. And I would say there
are probably as many different ways to celebrate personal and special days in
the calendar as there are people celebrating those days. And so the
calendar itself repeats every 400 years but you can choose how every year is
unique and special to you and I hope you have a very fantastic unique and
unnecessarily nerdy New Year. Fun fact: while this is something I made for my own
personal use and so you cannot buy it or give it to anyone for Christmas you can
give them my new book humble pi. Hear me out! Like, I'd really appreciate this.
Because if you order it we have a bonus gift we will actually give you two of
the chapters now and a certificate saying you bought it for someone so you
can give it to them before Christmas the book itself doesn't come out until the
7th of March 2019.. ooh I better mark that in. The 7th of March 2019. However if
you want to give it as a gift it's a bit lame to say you're gonna get some
generic book at a random date in the future...
here we go Thursday humble pi day. So this way you can get them the
certificate it says they're gonna get the full thing they can get two chapters
right now as an e-book so they can start reading they don't have to wait until
humble pi day the following year but on that date they will actually get the
book and if you order it through Maths Gear it'll be a first edition hardback
signed by me. If you order it through Amazon you can still get the free gift
I'll put a link to the website that Penguin has very kindly made for me if
you go there and put in your confirmation number you get all the
extra bonus stuff now and then Amazon will ship it in the new year. Amazon is
cheaper on Amazon if you get it from me I sign it and I've paid my taxes...
There you go. Just get it for everyone you know that would be amazing
and I really appreciate that. I mean that or of course go and get them 1985 Culture Club
calendar for a whole year of this much 80s.