Tagging timestamps; "proleptic"

John Cowan cowan at locke.ccil.org
Wed Oct 7 15:57:48 UTC 1998

Markus Kuhn wrote:

> Note that there is no reason for the tagging to show up in the bits of
> the value. The tagging can be done implicitly by naming your variables
> in software such that they make clear whether they are intended for UTC
> or TAI or whatever values.

It is not *necessary* for the timestamp-type to appear in the timestamp,
but doing so costs only a few bits and provides an excellent
defensive programming mechanism: it prevents you from attempting to
convert TAI timestamps to the local timezone, a meaningless activity.

> Question: What precisely does "proleptic" mean and where is it defined.
> The most official definition of the Gregorian calendar that I have to
> reference is ISO 8601, and it does not use the term "proleptic". My time
> and astronomy references to not define the term either.

The WWWebster dictionary defines "prolepsis" as:

	anticipation: the representation or assumption of
	a future act or development as if presently existing
	or accomplished [...]

"Proleptic" would be the standard adjectival form.  The HP MPE/ix
documentation (http://jazz.external.hp.com/src/year2000/dateintr.txt) says:

# All the date intrinsics follow what is called the "Proleptic Calendar".
# Stated in simple terms this calendar ignores the fact that calendars in
# different countries changed at different times (around the year 1753) .
# In other words, there are no lost days and there is no year 0.   This is
# similar to the calendar used by ALLBASE/SQL date/time functions.

The Julian Day count is based on a "proleptic Julian calendar", i.e.
the Julian calendar as if it was in use from 4713 B.C.E. to the

John Cowan	http://www.ccil.org/~cowan		cowan at ccil.org
	You tollerday donsk?  N.  You tolkatiff scowegian?  Nn.
	You spigotty anglease?  Nnn.  You phonio saxo?  Nnnn.
		Clear all so!  'Tis a Jute.... (Finnegans Wake 16.5)

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