Lots of good points here, but I think this _should_ be very simple:

>> The CF explanation of the calendar attribute is

>> In order to calculate a new date and time given a base date, base time and a 
>> time increment one must know what calendar to use. For this purpose we 
>> recommend that the calendar be specified by the attribute calendar which is 
>> assigned to the time coordinate variable.

The fact is that UTC and TIA are slightly different calendars -- as such users 
absolutely need to be able to specify which one is appropriate. Done.

So this proposal is a great (maybe theo ly, really) compromise be the 
specification we need and backward compatibility.

I am a bit confused by this, though:

gregorian_utc - <snip>
 the time values may not be fully metric.

huh? not entirely sure what "fully metric" means, but if, you have e.g.:

seconds since 1980-01-01T12:00:00

the the actually values in the array should fully represent monotonically 
increasing values, and time[i] - time[j-i] should always represent exactly the 
number of seconds that has elapsed.

what is non-metric about that???

BTW, this all is a really good reason to encode time in this way, rather than, 
say ISO datetime strings....

-CHB







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