[LEAPSECS] Crunching Bulletin B numbers (POSIX time)

Warner Losh imp at bsdimp.com
Tue Feb 22 17:16:51 EST 2011

On 02/22/2011 15:08, Tim Shepard wrote:

>> Having a known list of leap seconds let one recover TAI time from a cold

>> GPS receiver in a few seconds to a minute, rather than waiting ~20

>> minutes for the almanac to download. Many systems have their internal

> I thought GPS time had a constant offset from TAI of 19 seconds. So

> as soon as you have a GPS solution, you can find TAI. No table of

> leap seconds needed to accomplish that, as far as I know.


Should have put TAI and UTC here.

>> clocks in TAI, but also have requirements to report time in UTC to

>> users. Such a table would ensure these systems could come back online

>> quickly without having to pay the 20 minute penalty. While the system

>> may have already been down a little while when the swap is made, an

>> extra 20 minutes can kill reliability targets for the system.

> Hmm... maybe you could eliminate the need to report UTC time. Or

> have it report UTC time but have a flashing "aproximate" or

> "estimate" indicator next to the display which flashes for the

> first 20 minutes or so until it has received a complete copy of the

> almanac.

Nope. tried that when getting the spec approved. Approximate times
weren't allowed. UTC times were required. There was no way to indicate
approximate time in the user interfaces present (how do you blink a
5071A anyway :). The other systems that interfaced to ours had a fixed
format, and required UTC and not approximate UTC for a while and then a
possible jump in time to actual UTC.

Due to the complexity of the system, and the competing requirements, we
often would up waiting 20 minutes to do anything meaningful in those


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