In C, from
man 7 time:
UNIX systems represent time in seconds since the Epoch, 1970-01-01
00:00:00 +0000 (UTC).
A program can determine the calendar time using gettimeofday(2), which
returns time (in seconds and microseconds) that have elapsed since the
Epoch; time(2) provides similar information, but only with accuracy to
the nearest second.
You can use the
time() library function to get the epoch timestamp:
On 32-bit POSIX systems:
fprintf(stdout, "%u\n", (unsigned)time(NULL));
On 64-bit POSIX systems:
fprintf(stdout, "%lu\n", (unsigned long)time(NULL));