Update on CF and time

Hi-

Just to follow up on the issue that arose regarding dates.

Last week I said:
I will look into the unit parsing for this case though.  I'm
pretty sure that the underlying VisAD unit parsing only handles
UTC, GMT and Z as valid timezones. I'll see if we can ammend that.

The background is that the unit specification:

hours since 2003-12-01 00:00:00MDT

was not taken as a valid unit.  In looking at the description of
the ISO spec for time (ISO 8601) found at:

http://www.cl.cam.ac.uk/%7Emgk25/iso-time.html

parsing of a time zone like "MDT" can be problematic.  To quote
from that reference:

"There exists no international standard that specifies abbreviations for civil time zones like CET, EST, etc. and sometimes the same abbreviation is even used for two very different time zones. In addition, politicians enjoy modifying the rules for civil time zones, especially for daylight saving times, every few years, so the only really reliable way of describing a local time zone is to specify numerically the difference of local time to UTC".

Given that information, I am not going to make any changes to the
unit parsing to handle time zone specifications other than Z, UTC or
GMT (which the underlying VisAD unit parsing already supports).

As a side note, CF specifies that unit specifications should be udunits
compatible and at first glance udunits gives no error for the unit specification "hours since 2003-12-01 00:00:00MDT". In this case,
the MDT is ignored, so it takes it as GMT anyway.  udunits
does support the +/-hh:mm format and that is what should be used.

Don
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Don Murray                               UCAR Unidata Program
dmurray@xxxxxxxxxxxxxxxx                        P.O. Box 3000
(303) 497-8628                              Boulder, CO 80307
http://www.unidata.ucar.edu/staff/donm
        "Time makes everyone interesting, even YOU!"
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