HHMMSS = 10000 * hour + 100 * minutes + seconds
YYYYDDD = 1000 * year + day
where the year is 4-digits (1994, say, rather than just 94), and the
day is the Julian day-number (1,...,365 or 366).
Note that time intervals coded HHMMSS
will suffer INTEGER-overflow after a
little more than 24.5 years; however, for I/O API-3.1 or later,
a number of I/O API routines—CURREC(), CURRSTEP(),
JSTEP3(), LASTTIME(), NEXTTIME()—are carefully coded so as
to avoid such overflow, and can be used for time periods as long as
several thousand years. For I/O API files, the limiting factor is
INTEGER-overflow for the number of records. (For
files with a one-hour timestep, for example, this limits file-duration
to 89,478 years (which is sufficient for most applications:-).)
A time step sequence is a sequence of dates and times
{ [date(0),time(0)], [date(1),time(1)], ... [date[N],time[N] }
having a starting date and time [date(0),time(0)], a time increment dT,
and such that each [date(K),time(K)] is exactly dT away from
[date(K-1),time(K-1)]—for example, the following is a five-step
time step sequence starting at midnight January 2, 1980, with time step three hours:
{ 1980002:000000, 1980002:030000, 1980002:060000, 1980002:090000, 1980002:120000 }
Note that
each file has a time step
sequence (where by convention, time increment dT=0 means
that the data in the file is time-independent, and routines
like READ3() and WRITE3() which deal with
time-independent files ignore the date-and-time arguments. Note that
programs (e.g., m3tools programs) which control their run
timestep-sequence by specification of starting date&time, timestep,
and ending date&time are in practice not subject to
INTEGER-overflow problems; however, older ones controlled
by starting date&time, timestep, and duration may well be subject to
the above-mentioned 24.5-year limit. Unfortunately, this requires an
incompatible interface-change to fix...
Standard-Year and Standard-Week files are files for climatological and similar year-independent data. For these files, INTERP3() and possibly WKDAY() can be used to extract data for dates and times for the relevant day of year 0 according to the patterns
...
!! Standard-Year data:
!! Note: Fortran MOD() doesn't work "right"
!! for negative numbers:
IF ( JDATE .GT. 0 ) THEN
DAY = MOD( JDATE, 1000 )
ELSE
YEAR = (-JDATE)/1000 + 1
DAY = MOD( JDATE + 1000*YEAR, 1000 )
END IF
IF ( .NOT.INTERP3(<file>, <variable>, DAY, JTIME, ... )
...
!! Standard-Week data:
DAY = WKDAY( JDATE )
IF ( .NOT.INTERP3(<file>, <variable>, DAY, JTIME, ... )
...
Standard-year files should cover a period at least from
Year 1:Day 1 and at least through Year 1:Day 366,
and standard-week files should cover a period at least from
Year 1:Day 1 and at least through Year 1:Day 8.
Note that for monthly data valid at the middle of the month,
the starting date should be something like Year (-1),
Day 351, the time step 30.5 days (732 hours), and the ending date
Year 1, Day 16.
There are a number of utility routines available for manipulating dates and times, in addition to programs gregdate and juldate for converting back and forth between model-convention Julian dates and ordinary calendar dates. Note that for these utility routines, time increments may perfectly well be negative -- just make sure you keep the parts all positive or all negative; a time increment of -33000 means to step three and a half hours into the past, for example; the hours part is -3, the minutes part is -30, and the seconds part is 0. This way of representing dates and times is easy to understand and manipulate when you are watching code in the debugger (you don't have to turn the UNIXism "seconds since Jan. 1, 1970" into something meaningful for your model run, nor do you have to remember whether April has 30 days or 31 when your model run crosses over from April to May).
Utility routines for manipulating dates and times are the following:
Programs for manipulating dates and times are the following:
YYYYMMDD) and a
number of days D, and report the date
D days later.
Next Section: Calling from Fortran
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