Datetime is represented in, so called, external TAI64 format.
=> http://cr.yp.to/libtai/tai64.html TAI64
TAI stands for Temps Atomique International, the current international
real time standard. Unlike UTC, it takes leap seconds into account,
making it monotonous.
=> http://cr.yp.to/proto/utctai.html TAI

You can convert UTC to TAI by adding the number of leap seconds.

External TAI64 format is big-endian 64-bit number of TAI seconds passed
since 1970-01-01, plus 1<<62. TAI64N format adds 32-bit big-endian
number of nanoseconds (up to 999999999). TAI64NA adds another 32-big
big-endian part of attoseconds.

Shortest form *must* be used: if number of nanoseconds equals to zero,
then use TAI64 format.

Example representations:

1970-01-01 00:00:00 TAI             | 18 4000000000000000
1970-01-01 00:00:00 UTC             | 18 400000000000000A
1969-12-31 23:59:59 TAI             | 18 3FFFFFFFFFFFFFFF
1992-06-02 08:07:09 TAI             | 18 400000002A2B2C2D
1992-06-02 08:06:43 UTC             | 18 400000002A2B2C2D
1997-10-03 18:15:19 TAI             | 18 4000000034353637
1997-10-03 18:14:48 UTC             | 18 4000000034353637
2016-12-31 23:59:59 UTC             | 18 40000000586846A3
2017-01-01 00:00:00 UTC             | 18 40000000586846A5
2024-11-20 12:19:08 921772500ns UTC | 19 40000000673DD3E136F121D4
