[LEAPSECS] Bulletin C and all that

Rob Seaman seaman at noao.edu
Sun Jan 25 19:25:33 EST 2015

On Jan 25, 2015, at 1:03 PM, Stephen Scott <stephenscott at videotron.ca> wrote:

> Since UTC is defined by the IERS before 1972-01-01 "beginning_of_utc" is not appropriate.
> This is the beginning of integer leap seconds, not UTC.

Contributors to this list can always count on prompt fact checking ;-)  That said, the IERS came later than that, didn't it?

Interesting if UTC can indeed be said to have implemented two different mechanisms whose entire point was to keep Universal Time synchronized with Mean Solar Time.

> How about "leap_second_epoch" or if the term epoch is undesirable "leap_seconds_origin" labelled as "leap00"

Ok, I'll re-index to leap0 and have a new cname called origin.leapsec.com.  How's that?

On Jan 25, 2015, at 2:01 PM, Brooks Harris <brooks at edlmax.com> wrote:

> TAI is often also represented as a date-time but there is rarely a clear distinction made about what it means.

TAI is most naturally expressed as an unending tally of SI-seconds.  UTC as a sexigesimal fraction of a solar (synodic) day.  It is conversion between the two concepts that get us into trouble.  This DNS scheme might provide a small step toward letting them live together in greater harmony, and the tzdist standard a larger step addressing additional use cases.

> And, Rob, what, exactly, does "1972  1" mean in your Leap Seconds table? 1972-01-01T00:00:00 (TAI) or 1972-01-01T00:00:00Z (UTC)?

As PHK has observed, the essential concept of the IPv4 DNS leap second coding is to express the equivalent of Bulletin C.  I have always interpreted the independent variable of the IERS table as UTC.  Leap seconds are introduced at midnight UTC, not when TAI modulo 86400 equals zero.


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