In honor of this weekend being 1,600,000,000 (1.6 billion) seconds since the Unix epoch, I was wondering if anyone knows why January 1st 1970 was chosen?
According to Wikipedia,
The earliest versions of Unix time had a 32-bit integer incrementing at a rate of 60 Hz, which was the rate of the system clock on the hardware of the early Unix systems. The value 60 Hz still appears in some software interfaces as a result. The epoch also differed from the current value. The first edition Unix Programmer's Manual dated 3 November 1971 defines the Unix time as "the time since 00:00:00, 1 January 1971, measured in sixtieths of a second".
The User Manual also commented that "the chronologically-minded user will note that 2**32 sixtieths of a second is only about 2.5 years". Because of this limited range, the epoch was redefined more than once, before the rate was changed to 1 Hz and the epoch was set to its present value of 1 January 1970 00:00:00 UTC. This yielded a range of about 136 years, half of it before 1970 and half of it afterwards.
There's not really a mention of why it was chosen.
Honestly, it may be as simple as the logic below, but I'm curious if anyone has anything more definitive than a best guess.
- They wanted to use midnight January 1st at GMT because it is the start of the year in a "neutral" timezone.
- They used 1971 first because they could only express ~2.5 years of time when using 60 Hz intervals
- They used 1970 when they updated to 1 Hz intervals to round it to a "nicer" number (1970 has an additional zero) and is close to the original epoch they used before.
One thing that sticks out is why not use the year 2000 instead of 1970? It is "nicer" than 1970 since it has more zeroes and is not so far in the future that the ~136 years of time that could be expressed before it is prohibitive (e.g., if you chose 2100 you could only express as far back as ~1964). (To be clear, I am not asking why not use 2000 since that makes the question more subjective, I am just pointing out that 1970 is not some immediately obvious special number.)
For an example of a timekeeping format that uses a more "meaningful" epoch there are (Time-based) UUIDs. They use midnight October 15th 1582 as the epoch because it was when the Gregorian calendar began being used.