FW: Definition of time_t changed from signed to unsigned...

John Cowan jcowan at reutershealth.com
Fri Aug 20 05:20:17 UTC 2004

Paul Eggert scripsit:

> > A count of UTC seconds since the Epoch is the same as a count of TAI
> > seconds
> Only if you are talking about broken-down labels for time.  But I was
> talking about time expressed as a count of seconds.  For example, the
> two adjacent real-time seconds with broken-down labels 1998-12-31
> 23:59:60 and 1999-01-01 00:00:00 UTC have the same count-of-seconds
> since the epoch.

No, I don't believe so.  The two adjacent seconds you mention have the
same _Posix_ time, but the number of elapsed UTC seconds = TAI seconds =
SI seconds since the Epoch is not the same; ergo, on an Olsen-right system
the values returned by gmtime will be different for these two labels.

> Since TAI ticks along uniformly, and UTC-TAI jumps backwards by 1
> second at the end of the inserted leap second, it logically follows
> that UTC also jumps backward by 1 second at the same moment.

Both TAI and UTC tick along uniformly at a rate of 1 SI second per second
at all times (as opposed to other time scales which do not).  It is
simply that the broken-down time labels of UTC do something unusual at
a leap second, whereas those of TAI do not.  The statement that TAI-UTC
changes applies to broken-down time labels, not to elapsed time.

(I suspect that our disagreement is primarily or even entirely

"They tried to pierce your heart                John Cowan
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