[CF-metadata] udunits handling of fuzzy time units

From: Jonathan Gregory <j.m.gregory>
Date: Wed, 16 Mar 2011 17:15:04 +0000

Dear John

> Suppose we added the UTC_Calendar to CF, which tracks leap seconds
> etc. So if one had the time coordinate "days since 1800-01-01" with
> values = "0,1,2,3..." and we need the resulting coordinates to be
> "1800-01-01", "1800-01-02", "1800-01-03", "1800-01-04",.... which in
> this calendar gives an uneven number of seconds between coordinates.
>
> So all timeUnits (except seconds) now mean "increment the calendar
> field", not "add x secs to base", that is, its calendar dependent if
> any timeUnit implies a fixed number of seconds.

In fact that also raises the same problem as I did in my last email. If a
second is removed, there will be some occasions when adding 1 day to a given
time will not have an obvious meaning, because the corresponding time a day
later may not exist in the calendar. A rule is needed to resolve that.

> In that case, then fractional values may not make sense(?)

It will depend on the calendar, presumably. In the 360_day calendar, all the
units of time have fixed length. In the UTC calendar, none of them do except
seconds, apparently. Fractions are OK with fixed-length units. In the 360_day
calendar, 0.5 year means 180*86400 seconds exactly.

Cheers

Jonathan
Received on Wed Mar 16 2011 - 11:15:04 GMT