[LEAPSECS] any other parties?
imp at bsdimp.com
Mon Jul 9 11:13:45 EDT 2012
On Jul 9, 2012, at 9:06 AM, Rob Seaman wrote:
> On Jul 9, 2012, at 7:11 AM, Warner Losh wrote:
>> There's four different Unix implementations of leap seconds
>> (1) Repeat the first second of the next day.
>> (2) Repeat the last second of leap day.
>> (3) Freeze time
>> (4) slew it in over many hours.
> Just to echo a well traveled talking point here, note that none of these is actually compliant with UTC. Actual UTC compliance would be simpler to implement. UTC is just an overlay on an unending stream of TAI seconds. The reality is there is no leap second.
Except that isn't POSIX time_t compliant, alas. That's the other variation I forgot, which is to use the "right" timezone files, which also have their own set of problems for long-running applications (a variation on getting the UTC leap second tables problem). I've gone on at length in other forums why this is clever, but not a complete solution.
Also, a never ending stream of TAI seconds is easy to count, but hard to convert to UTC since you need a leap second table to do that. This can present problems to applications that need to present a UTC time to the outside world that have been off for a while. GPS can give you the current TAI time very quickly, but cannot give you the UTC time until it has downloaded the almanac (especially if the device has been off > 6 months).
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