Re: [LEAPSECS] Comparing Time Scales

From: Warner Losh <imp_at_BSDIMP.COM>
Date: Fri, 03 Feb 2006 22:01:39 -0700 (MST)

From: "Daniel R. Tobias" <dan_at_TOBIAS.NAME>
Subject: Re: [LEAPSECS] Comparing Time Scales
Date: Fri, 03 Feb 2006 21:27:54 -0500

> On 3 Feb 2006 at 15:37, James Maynard wrote:
>
> > Thanks, guys, for your feedback. Here's another iteration.
>
> It would seem that PTP is actually counting from 1970-01-01 00:00:00
> in TAI rather than UTC; the subsequent values of that scale reflect
> the full TAI-UTC offset, not just the part of it that occurred after
> 1970. For instance, the PTP value for 1972-01-01 00:00:00 UTC
> consists of the number of seconds in the last two (non-leap) years
> plus 10 seconds. The actual UTC value at 1970-01-01 reflected the
> variable-length "rubber seconds" of the time, so the TAI-UTC offset
> then was some fractional value that was less than 10 but greater than
> 0.

My company produces measurements that are tagged with UTC time and TAI
time. The UTC time is the broken down time, while the TAI is the
number of seconds since 1972 plus a contant. This gives the same
effect as the number of PPS pulses since 1970 without having to worry
about rubber seconds between 1970 and 1972 when just under 2 seconds
accumulated in the TAI-UTC offset. It works out so that if you take a
time today, and add the TAI-UTC offset (or is that subtract), you get
a number that you can convert using the POSIX routines into a UTC time
for all seconds execpt leap seconds.

Warner
Received on Fri Feb 03 2006 - 21:03:08 PST