[LEAPSECS] Toasting Unix timestamp 1234567890

Magnus Danielson magnus at rubidium.dyndns.org
Fri Feb 13 20:42:48 EST 2009

Zefram skrev:

> Neal McBurnett wrote:

>> So I'll also celebrate the passing of 1234567890 UTC seconds

>> since January 1 1970, 00:00, which is 24 seconds earlier.

>

> To be precise, you must also include the leap at the end of 1971,

> of 10775800/100000003 UTC seconds. That's 0.107758 TAI seconds, at

> the then-prevailing rate of 1.00000003 TAI seconds per UTC second.

> (All these numbers exact.)

The reference epoch at MJD 40587. This can be used with this table
http://hpiers.obspm.fr/eop-pc/earthor/utc/TAI-UTC_tab.html
to convert into the TAI-UTC difference.

1966 Jan. 1 - 1968 Feb. 1 4.313 170 0s + (MJD - 39 126) x 0.002592s
1968 Feb. 1 - 1972 Jan. 1 4.213 170 0s + ""

Thus, the TAI-UTC difference was 4.213170 + (40587-39126) x 0.002592s =
8.000082 s.

Quite close but no cigar to 8 s sharp. 82 us off.

> It is somewhat more meaningful to treat Unix time as based on vague UT,

> rather than specifically UTC. The Unix time 1234567890 will, to within

> a second, mark 1234567890 UT1 seconds since 1970-01-01T00:00:00 UT1.

Well, it depends on what you actually mean by "Unix time". The original
definition puts the reference epoch to 00:00:00 GMT. Then you can make a
pick on what seconds you want to use... and then you have the Posix
time... which maps UTC to time_t...

Actually, if we interprent GMT as UT1, then the above calculation is
again wrong as it gives the UTC but not UT1... so we need to dig up some
pretty old Circular B papers... which does not seem to be online...

So depending on which interpretation you choose... I see some 3-4
different times occuring. The spread amongst them is about 26 s or so.

Cheers,
Magnus