I wrote a near-trivial "timestamp" command to make it easier to do time arithmetic (handy for scheduling a doctor's appointment a 90-day medicine supply from now).

TZ=udt timestamp
119 11 04 21 50 06 18204 1572904206 Mon Nov  4 21:50:06 2019
TZ=udt timestamp 0
70 01 01 00 00 00 0 0 Thu Jan  1 00:00:00 1970
TZ=udt timestamp 500000000
85 11 05 00 53 20 5787 500000000 Tue Nov  5 00:53:20 1985
TZ=udt timestamp 1000000000
101 09 09 01 46 40 11574 1000000000 Sun Sep  9 01:46:40 2001
TZ=udt timestamp 1500000000
117 07 14 02 40 00 17361 1500000000 Fri Jul 14 02:40:00 2017
TZ=udt timestamp 2000000000
133 05 18 03 33 20 23148 2000000000 Wed May 18 03:33:20 2033
TZ=udt timestamp 2500000000
149 03 22 04 26 40 28935 2500000000 Mon Mar 22 04:26:40 2049

Not likely I'll live to see 2500000000. I surely won't live to see a half-billion years.

On Mon, Nov 4, 2019 at 4:11 PM Dave Horsfall <dave@horsfall.org> wrote:
On Tue, 5 Nov 2019, Dave Horsfall wrote:

> UNIX was half a billion (500000000) seconds old on Tue Nov 5 00:53:20
> 1985 GMT (measuring since the time(2) epoch).

Aarrgghh!  The subject should read "seconds", of course (too much blood in
my coffee stream).

-- Dave