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Comment by euler_angles | Hacker News Reader

I am curious, when did the decision to make GPS not follow leap seconds occur? had you asked me before I read your comment, I would have said that it doesn't honor leap seconds and never has.

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Yeah, so GPS was designed to not apply leap seconds from the start, which is important because there's a field built into the protocol to send the current offset. The reason I describe it as "no longer applying" is because the GPS time is monotonic from UTC at an arbitrary starting point, so it doesn't match up with any of the strictly monotonic time bases either - that is, GPS time is monotonic but does not match the agreed upon international monotonic time standard, TAI, because GPS time has some of the UTC leap seconds applied.

TAI is also, of course, fundamentally just a divergence from UTC caused by not applying leap seconds. However, formally, UTC is defined as TAI plus or minus the leap second offset in order to minimize divergence from UT1 (which is a smoothed astronomical time base rather than monotonic). GPS time is TAI plus or minus a different offset caused by some of the leap seconds being considered.

There is a basis for this, LORAN did the exact same thing but with an earlier divergence point.

Ah, this makes sense. I deal with some data analysis that has times referenced to the GPS epoch, but also includes all the leap seconds added from the GPS epoch through September 19, 2013. So I've spent a fair amount of time dealing with this but haven't had to get as deep as you have. thanks for the detailed reply

Atomic clock timing services that existed before GPS didn't use leap seconds either. GPS time is related to TAI time, which is based on absolute elapsed time so it slowly drifts away from UTC when leap seconds get added. IIRC, GPS time is just TAI/TT time with a different epoch.

You're correct it never has. GPS time started 6 Jan 1980 and has ignored every leap second since.

The painful fact is that it does include the leap seconds before January 6th 1980. Makes it messy to convert between UTC and GPS time.

I think all the online converters I found a couple of years ago failed to get this right.

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