The Lotus Epoch

If you wanted to set a landmark, you could pick any date, but a nice round number seems reasonable. Let's say, for example, January 1st, 1900. From there, it's easy to just add and subtract numbers of days to produce new dates. Oh, but you do have to think about leap years. Leap years are more complicated- a year is a leap year if it's divisible by four, but not if it's divisible by 100, unless it's also divisible by 400. That's a lot of math to do if you're trying to fit a thousand rows in a spreadsheet on a computer with less horsepower than your average 2019 thermostat.

So you cheat. Checking if a number is divisible by four doesn't require a modulus operation- you can check that with a bitmask, which is super fast. Unfortunately, it means your code is wrong, because you think 1900 is a leap year. Now all your dates after February 28th are off-by-one. Then again, you're the one counting. Speaking of being the one counting, while arrays might start at zero, normal humans start counting at one, so January 1st should be 1, which makes December 31st, 1899 your "zero" date.

(...)

Time marches on. Excel needs to have macros, and the thought is to bolt on some of the newfangled Object-Oriented Basic folks have been experimenting with. This is a full-fledged programming language, so there's already an assumption that it should be able to handle dates correctly, and that means counting leap years properly. So this dialect of Basic doesn't think 1900 is a leap year.

This macro language thinks that 60 days after December 31st, 1899 is March 1st.

No problem. Move your start date back one more, to December 30th, 1899.

MUMPS/M/Cache counts the number of seconds since Dec. 31, 1840. When it was first defined, it had to handle medical records, and, at the time, the oldest date in the dataset was someone born in 1841.

“Nothing – and I mean nothing – in IT takes less than 80 hours, and whatever you think it’ll actually take, multiply it by 20, and tell management that.”