Daan Leijen writes:
clock: measures passage of absolute time (time difference). Best available unit is (TAI) SI seconds (ie. cesium atom oscillations)
calendar: gives time *stamps* to human related notion of time. Can be used to calculate calendar time differences, ie. human notions of time difference, like days and months, and yes, *calendar* seconds (ie. not absolute SI seconds).
TAI is a clock. Sometimes they give it as date, but really, it is just SI-seconds since some epoch (specified as an UTC calendar date!)
This could be correct, but my understanding is that TAI and UTC are both calendars that are based on the same clock. The difference is that the UTC clock/calendar conversion requires a leap-second table and the TAI conversion does not. The problem is that NTP is based on a different clock where some seconds are longer than others.
Now what I understand about implementation:
- gettimeofday() returns the number of seconds since epoch (a UTC date)
What kind of seconds does it return? Well, (A) sometimes these are just SI-seconds (which means we can derive TAI from that), but sometimes (B) it returns UTC-seconds -- ie. the absolute SI-second count, plus/minus any leap seconds that have occurred.
I think you mean "POSIX-seconds" instead of "UTC-seconds" here.
Otherwise, (A) and (B) seem identical to me.
--
David Menendez