[CF-metadata] time dimension and calendar for an ice-sheet model output

[Constantine Khroulev]

Hello,

I am updating PISM to CF-1.4, and I have a question about the  
reference date and calendar.
In our context
1) time in the output file denotes "seconds (or years) relative to  
present", i.e. it for paleoclimate runs it is negative (and is used to  
extract appropriate temperature and sea-level offsets from ice-core  
data, for example) and for runs "into the future" it is positive.
2) a year is always 3.15569e+07 seconds.

I might be wrong, but it seems that 1) suggests that any particular  
"real" reference date will be... well... unfitting, and 2) suggests  
that standard calendars are not applicable here either.

It is appropriate to use the units string of "years since 1-1-1 0:0:0"  
or similar, following Example 4.6?

Should I use calendar="none", or should I define a custom calendar?

What would you recommend?

Thank you!

-- 
Constantine Khroulev
PISM (www.pism-docs.org) Developer/Maintainer
University of Alaska Fairbanks