# Date and Time Systems

There are/have been various methods of timekeeping.

The 12-hour system is 1-based. There is also a 24-hour system that is 0-based. In the 0-based 24 hour system the time formerly known as 12 o’clock midnight became instead 00:00.

There are a number of workarounds for the fact that there is no year 0 between 1 BCE and 1 CE. One way is the Unix time method, just pick an arbitrary date for day zero. Or do what most people do and start using a 0-based system implicitly. For the purpose of celebration just assume the decades start with the year ending with zero and don’t worry about the details.

Anyone that wants to keep using the other system is free to do so.

Also discussed 12-hour clocks and modulo arithmetic here: Radar Second -Trace / Unambiguous Range - #27 by Kennebec_Captain

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on paper yes but when you digitze it, time gets made up with bits that need to fit the computer hence the fudge when you convert it back for humans to read it.
Days months years decades and centuries make sense to us but its a all the same for a computer except when its programmed in 2 digit years…

Not to mention the GPS system where the date (week number) rolls back to 0 every 1024 weeks (and much mayhem ensues!)

and
the earth doesnt quite have a 24hr day so that has to be edited every few years.
God should have been metric and all this would go away…lol

Hold on, what were we talking about? When a decade starts? First someone has to say what year system we’re talking about.

GPS is based on the Julian year, which is based on the metric second, which is based on cesium. The display on all GPS units currently subtracts 18 seconds to align with sidereal time which is close to our UTC. Same goes for the astronomical almanacs we use for navigation. (This was a pain when making a nautical almanac in Excel using planetary orbital elements… those currently started on 01 Jan 2000.) Which means when someone uses GPS time (or International Atomic Clock Time, delta 37 seconds) for navigation they will plot themselves a great distance off.

In any case the Julian year started on a noon back in 4713 BCE while TIA started in 2005. UTC keeps changing every time a leap second is added or subtracted and that damn sidereal year starts on what was the First Point of Aries back in about 130 BCE…

February 4, 2022 is day of year 35 in GPS Week 2195.

If you want to be accurate the clocks in GPS satellites gets updated all the time as they are moving in space and trying to orbit the earth so they speed up and slow down effecting the time.
Thats one of the corrections you pay for with subscription services, the vendor sends the correct edit to you plus the orbit location
1 nano second is 300mm error on the ground

Then you have zero longitude which is not at the line in Greenwich any more…
Thank god for radar or I could never find a port.

I agree with this (in context) but if we want to get pedantic about this (and we do of course) than in practice people can usually determine what system is in use from the context.

If someone is invited to a New Year’s (or new decade) party than “what system are you using?” might cause them to get disinvited.

I just learned that the other day.

" In fact, it’s out by about 100 metres."

It’s somewhat ironic that the line is “concrete” or “carved in stone”.

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Only slightly off-topic … no matter what time system you prefer, MOL has opened an online shop to sell clocks and goodies removed by breakers.

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Bowditch has an entire chapter named “Time” which I’m familiar with and I’ve been reading some historical stuff, for example a book about Copernicus but I couldn’t wrap my head around how the ancient astronomers could have figured out the things they did.

So I built a big sundial from wood stickers which we have everywhere around here (for drying lumber).

The gnomon is about 2 meters tall.

I realize how sad this looks, both aesthetically and technically but here’s a photo:

I took most of the hourly sticks out so I could mow the lawn but the stick on the left was placed on winter solstice at LAN and one for summer solstice is to the right.

I learned a couple things, people are impressed when the shadow falls as predicted, what path the shadow follows and why monoliths are build from stones and not sticks.

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Year zero spotted in the wild.

From The Dawn of Everything by Graeber and Wengrow.