Year2000

[William Pence]

I support the Year200 proposal with 2 clarifications as listed below.

-Bill Pence

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>                           6) Transition
>  
> FITS readers must continue to interpret the old format, as a twentieth
> century date, forever.  Readers should be altered as soon as possible
> to cope with the new format.  In order to give adequate time for the
> major package writers to revise their software, FITS writers should commence
> writing the new format between 1998-01-01T00:00:00 and 2000-01-01T00:00:00.

It would be less ambiguous if the last sentence were changed to
something similar to:

"In order to give adequate time for the major package writers to revise
their software, FITS writers should commence writing the new format as
soon as possible after (but not before) 1998-01-01T00:00:00."

>                     7) Appendix: Suggested time scale specification
> 
>        [Note: this appendix is not part of the formal DATExxxx agreement]
> 
> 7.1) Use of the keyword TIMESYS is suggested as an implementation of the time
>    scale specification; it is currently in use as such in the RXTE mission
>    archive.  It sets the dominant time system for time-related keywords
>    and data in the HDU (i.e., it does not preclude the addition of keywords
>    or data columns that provide information for transformations to other
>    time scales, such as sidereal times or barycenter corrections).
>    Initially, officially allowed values are:
>       UTC  Coordinated Universal Time; defined since 1972.
>       UT   Universal Time, equal to Greenwich Mean Time (GMT) since 1925;
>             the UTC equivalent before 1972;
>             see: Explanatory Supplement, p. 76.
>       TAI  International Atomic Time; "UTC without the leap seconds";
>             31 s ahead of UTC on 1997-07-01.
>       ET   Ephemeris Time, the predecessor of TT; valid until 1984.
>       TT   Terrestrial Time, the IAU standard time scale since 1984;
>             continuous with ET and synchronous with (32.184 s ahead of) TAI.
>       TDT  Terrestrial Dynamical Time; =TT.
>       TDB  Barycentric Dynamical Time.
>       TCG  Geocentric Coordinate Time; runs ahead of TT since 1977-01-01
>             at a rate of approximately 22 ms/year.
>       TCB  Barycentric Coordinate Time; runs ahead of TDB since 1977-01-01
>             at a rate of approximately 0.5 s/year.

The HEASARC has a large archive of FITS files containing the TIMESYS
keyword with other values, such as  'MJD', or '1980.0'.  I would like
to be reassured that the current proposal does not invalidate the past
and continued use of the TIMESYS keyword in this way by the HEASARC.  I
would assume that this is the case since this Appendix is not part of
the formal agreement.

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