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Comment by DrBazza | Hacker News Reader

There's a certain exchange out there that I wrote some code for recently, that runs on top of VAX, or rather OpenVMS, and that has an epoch of November 17, 1858, the first time I've seen a mention of a non-unix epoch in my career. Fortunately, it is abstracted to be the unix epoch in the code I was using.

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Apparently the 1858 epoch comes from an astronomy standard calendar called the Julian Day, where day zero was in 4713 BC:

https://www.slac.stanford.edu/~rkj/crazytime.txt

To make these dates fit in computer memory in the 1950s, they offset the calendar by 2.4 million days, placing day zero on November 17, 1858.

It’s called the modified Julian day (MJD). And the offset is 2,400,000.5 days. In the Julian day way of counting, each day ended at noon, so that all astronomical observations done in one night would be the same Julian day, at least in Europe. MJD moved the epoch back to midnight.

https://en.wikipedia.org/wiki/Julian_day

There's an old Microsoft tale related to the conflict between Excel's epoch of Jan 1 1900 vs Basic's Dec 31 1899:

https://www.joelonsoftware.com/2006/06/16/my-first-billg-rev...

Another common computing system to be aware of: the Windows epoch is 1-Jan-1601.

The classic MacOS, Apple's HFS file system (also used in OS X), and PalmOS all had en epoch of January 1, 1904.

The macOS/Swift Foundation API NSDate.timeIntervalSinceReferenceDate uses an epoch of January 1, 2001.

edit: Looks like Wikipedia has a handy list https://en.wikipedia.org/wiki/Epoch_(computing)#Notable_epoc...

PostgreSQL internally uses a 2000-01-01 epoch for storing timestamps.

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