A computer doesn't keep a date. It keeps one count: the seconds since a starting line, midnight UTC on 1 January 1970. Every date you see is worked out from that count. On a laptop, as I made this, the count was 1,792,046,738.

The starting line was chosen, and it moved. The first Unix manual (Ken Thompson and Dennis Ritchie, Bell Labs, 3 November 1971) counted "the time since 00:00:00, Jan. 1, 1971, measured in sixtieths of a second", and noted that "2**32 sixtieths of a second is only about 2.5 years." In 1972 the line moved to 1 January 1972; by 1973 the manual said "the time since 00:00:00 GMT, Jan. 1, 1970, measured in seconds."

So when a gadget loses its count, or a file's date is blank, it reads zero, and zero is midnight on 1 January 1970 (in New York, still 7 pm on New Year's Eve 1969). I asked a laptop which date count zero is, and it said the same. A 1970 date means the count was lost, not that the file is from 1970: set the clock again, or let it sync, and new dates come back right.

The count has its own cliff. Kept in 32 bits, it runs out at 03:14:07 UTC on 19 January 2038 and jumps back to 13 December 1901 (I ran it on a laptop). A laptop today counts in 64 bits: by our arithmetic that lasts about 292 billion years.

Your computer doesn't know what day it is. It only knows how many seconds since 1970.

What's the oldest date you've ever seen on a file?

Sources: K. Thompson and D. M. Ritchie, UNIX Programmer's Manual, First Edition, Bell Labs, 1971 (sys time); UNIX Third and Fourth Edition manuals, time(II), 1972 and 1973; POSIX, IEEE Std 1003.1-2024, Epoch and Seconds Since the Epoch; GNU C Library 2.34 release notes, 2021; our own tests on a laptop, 2026.
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