[LEAPSECS] Toasting Unix timestamp 1234567890

[Magnus Danielson]

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