CF Convention forTime

Time variable

• must include units (see format)
• has no default value
• time coordinate and the datetime are interconvertible given the calendar
• must comprise units of measure, “since”, and a reference datetime. (“days sinced 1990-01-01”)
  • acceptable unit of measures (day (h), hour (hr, h), minute (min), and second (sec, s). Plurals are acceptable
• leap seconds should be properly dealt but often not the case. If using standard, proleptic_gregorian, and julian for calenar, one can add units_metadata to indicate “leap_seconds”

time units

double time(time) ;
  time:axis = "T"; // optional
  time:standard_name = "time" ; // optional
  time:units = "days since 1990-1-1 0:0:0" ; // mandatory

Example of how adding leap second information

  float time_stdnone ;
    time_stdnone:standard_name = "time" ;
    time_stdnone:long_name = "Model data with no leap seconds" ;
    time_stdnone:calendar = "standard" ;
    time_stdnone:units = "seconds since 2016-12-31 23:59:58" ;
    time_stdnone:units_metadata = "leap_seconds: none" ;
  float time_stdutc ;
    time_stdutc:standard_name = "time" ;
    time_stdutc:long_name = "Model data with leap seconds or obs data with accurate UTC" ;
    time_stdutc:calendar = "standard" ;
    time_stdutc:units = "seconds since 2016-12-31 23:59:58" ;
    time_stdutc:units_metadata = "leap_seconds: utc" ;

Time Calendar

• CF calendar refers to datetime, not just date.
• calendar standard: default UDUNITS. A deprecated alternative name is gregorian.
• other calendar categories are proleptic_gregorian, julian, and utc. Read more at cf-convention