[CF-metadata] date and time

From: Jonathan Gregory <j.m.gregory>
Date: Fri, 24 Oct 2008 20:52:28 +0100

Dear John

> time_from_device: time reported by some device (data source)

Somehow this seems a bit unsatisfying to me. It doesn't really suggest that
this time might not be true time. Given your definition, why not call it
reported_time, or maybe indicated_time. Your original sensor_time has the
different advantage that we have another stdname mentioning "sensor".

> (I just changed the form of this name so that it is collocated
> with the other time variables)

I sympathise, but I think we should deal with that problem by having better
tools for displaying and searching the table. There are many groups of
related stdnames that are not adjacent in alphabetical order.

> The following three can each have an attribute pointing to the
> variable that determines the time sequence; for example, the attribute
> could name another source of time values, and the current year is
> established by those values. The units in all 3 are the same as the
> units for time.
>
> time_since_start_of_year: amount of time elapsed since the current
> year began
> time_since_start_of_day: amount of time elapsed since the current day
> began

If you have other time variables which indicate the day and year, these
would be redundant, wouldn't they? You can deduce them from the time variable,
especially if it is in days since midnight on 1 Jan in some year. I'm always
worried about redundancy meaning possible inconsistency.

> time_since_start_of_mission: amount of time elapsed since the current
> mission began

If the time variable has the start of the mission as the reference in its
units, it serves both purposes. That would be an informal extra convention.

To deduce the time since the start of the mission from a time variable, you
need to know when the mission started. So perhaps you could store a variable
for mission_start_time. That would be analogous to forecast_reference_time,
and would be scalar. I feel more comfortable with that than with time since
start of mission, because the latter is an array with a constant offset from
the actual time, so again this would be redundant. However if the actual time
itself is not stored in a variable, the time since the start of the mission
would be independent, and I'd be happy with it. As a slightly snappier but I
think equally clear name, what about mission_elapsed_time, which is what NASA
call it?

Best wishes

Jonathan
Received on Fri Oct 24 2008 - 13:52:28 BST