Hi all,
My understanding is that leap-seconds are inserted so that the
calendar remains (almost) perfectly synchronized with the seasons. If,
under the CF convention, leap seconds are ignored and time is recorded
as, say, "days since 2007", and two times were recorded, 0.5 and 365.5,
those times would correspond to 12 noon of Jan 1, 2007 and 12 noon of
Jan 1, 2008. If, however, the CF calendar were required to be truly
realistic, the leap second which in fact was inserted at the end of 2007
would mean the second time should be 11:59:59 of Jan 1, 2008.
I am pretty sure that the vast majority of datasets stored under the CF
convention will not take into account leap seconds and will record times
assuming they are not there. I think it is unrealistic to assume users
will adhere to a truly realistic calendar when recording their times.
In the above example, if the measurements were made at noon of each day,
then you would have to record times of 0.5 and 365.5+1./(24*60*60) =
365.500011574 (rather than 0.5 and 365.5).
I therefore think the CF convention should specify that the times
recorded in files should be interpreted according to the specified
calendar, but assuming that leap-seconds have been ignored.
cheers,
Karl
On 8/19/11 9:28 AM, John Caron wrote:
On 8/19/2011 7:48 AM, Lynnes, Christopher S. (GSFC-6102) wrote:
On Aug 19, 2011, at 7:54 AM, Jon Blower wrote:
Hi Chris,
Does the calendar system usually define whether leap-seconds are taken into
account or not?
Generally speaking, I don't believe so, if only because calendar systems
usually go down to the day granularity. Seconds-based doordinate time
standards, such as UTC and TAI do, because they have to. Julian date (NOT the
day of year many associate with the name) is an exception, with its fractional
dates.
In other words, given knowledge of which calendar system is in use, could a
library make the correct calculation? Or is other information needed too?
In my limited understanding, a calendar system might track leap seconds
or not. If it does, then all calendar fields (except seconds and
millisecs) would be variable length. But for any given calendar, the
interval (in seconds) between two valid dates would be well defined. Of
course, these intervals will differ between calendars.
Therefore, one can talk about a "correct" implementation of a calendar
system, and one can talk about the difference between two calendar
systems, one of which is the correct one to use in some context.
Otherwise, im not sure there really is a "correct calculation", or at
least that might be a confusing way to state the issue.
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