Introduction

The NCEP utility, ndate, will add or subtract hours from a date code. Wgrib2 already has routines to do these calculations. So instead of porting the ndate fortran code and the NCEP libraries, how about a simple-to-write option that would allow wgrib2 to do the same calculation (and more)? In my case, installing wgrib2 is a probably a given. So adding -ndate saves me time.

The ndate utility allows you to add or subtract a integer number of hours from a YYYYMMDDHH date code, The -ndate option allows you to add or subtract an integer number of minutes, hours, days, months or years from a date code. The output format has the precision necessary for the input date code and offset. Sure you could do all this with the gnu date program but the -ndate option is easier to use.

The feature of the ndate utility that is not in wgrib2, is the ability to print the current UTC date code when there is no argument. Of course, you can do the same by "date -u +%Y%m%d%H". For users of the ndate utility, note that the order of arguments are reversed between the utility and wgrib2 option.

The -ndate option is an odd option because it does its output in the initialization phase. In order to to trigger an "missing input file" error, you need to run wgrib2 on a valid file. For linux/unix systems, using the file /dev/null is a convenient, always present file.

Usage

-ndate DATE OFFSET
       DATE = YYYY, YYYYMM, YYYYMMDD, YYYYMMDDHH, YYYYMMDDHHmm, YYYYMMDDHHmmss
       OFFSET=(integer)(yr|mo|dy|hr|mn)
                integer can be positive or negative
                yr = year  (0000..9999)
                mo = month (01..12)
                dy = day   (01..31)
                hr = hour  (00..23)
                mn = minute (not to be confused with month)  (00..59)

      The output format has the precision to reflects the maximum precision of 
      the date code and the offset

     Priority of the output format of the date code
   output date code: YYYYMMDDHHmmss      if input date code is YYYYMMDDHHmmss
   output date code: YYYYMMDDHHmm        if input date code is YYYYMMDDHHmm   or OFFSET is in minutes (mn)
   output date code: YYYYMMDDHH          if input date code is YYYYMMDDHH     or OFFSET is in hours (hr)
   output date code: YYYYMMDD            if input date code is YYYYMMDD       or OFFSET is in days (dy)
   output date code: YYYYMM              if input date code is YYYYMM         or OFFSET is in months (mo)
   output date code: YYYY                if input date code is YYYY           or OFFSET is in years (yr)

Example

$ wgrib2 /dev/null -ndate  2016010212 -6hr
2016010206

See also: -ndates