Further details Where time is a Variable (Independent, Dependent or Auxiliary), please use the guidelines given for CF-compliant NetCDF data for time. The CF document lists a set of accepted unit types that can be used as set out by the UDUNITS software package, these are listed in Appendix I.
The following full explanation is copied from the CF-metadata web site: Variables representing time must always explicitly include the units attribute; there is no default value. The units attribute takes a string value formatted as per the recommendations in the Udunits package. The following excerpt from the Udunits documentation explains the time unit encoding by example:
The acceptable units for time are listed in Appendex I. The most commonly used of these strings (and their abbreviations) includes day (d), hour (hr, h), minute (min) and second (sec, s). Plural forms are also acceptable. The reference time string (appearing after the identifier since) may include date alone; date and time; or date, time, and time zone. The reference time is required. A reference time in year 0 has a special meaning (see CF documentation).
seconds since 1992-10-8 15:15:42.5 -6:00
indicates seconds since October 8th, 1992 at 3 hours, 15 minutes and 42.5 seconds in the afternoon in the time zone which is six hours to the west of Coordinated Universal Time (i.e. Mountain Daylight Time). The time zone specification can also be written without a colon using one or two-digits (indicating hours) or three or four digits (indicating hours and minutes).
We recommend that the unit year be used with caution. The Udunits package defines a year to be exactly 365.242198781 days (the interval between 2 successive passages of the sun through vernal equinox). It is not a calendar year. Udunits includes the following definitions for years: a common_year is 365 days, a leap_year is 366 days, a Julian_year is 365.25 days, and a Gregorian_year is 365.2425 days.
For similar reasons the unit month, which is defined in udunits.dat to be exactly year/12, should also be used with caution.
Appendix I: Accepted units of time in the UDUNITS software package
# UNITS OF TIME # day 8.64e4 second # exact hour 3.6e3 second # exact minute 60 second # exact s second sec second shake 1e-8 second # exact sidereal_day 8.616409e4 second sidereal_hour 3.590170e3 second sidereal_minute 5.983617e1 second sidereal_second 0.9972696 second sidereal_year 3.155815e7 second # Interval between 2 successive passages of sun through vernal equinox # (365.242198781 days -- see # http://www.ast.cam.ac.uk/pubinfo/leaflets/, # http://aa.usno.navy.mil/AA/ # and http://adswww.colorado.edu/adswww/astro_coord.html): tropical_year 3.15569259747e7 second lunar_month 29.530589 day common_year 365 day # exact: 153600e7 seconds leap_year 366 day # exact Julian_year 365.25 day # exact Gregorian_year 365.2425 day # exact sidereal_month 27.321661 day tropical_month 27.321582 day d day min minute hr hour h hour fortnight 14 day # exact week 7 day # exact jiffy 0.01 second # believe it or not! jiffies jiffy # assumed plural spelling year tropical_year yr year a year # "anno" eon 1e9 year # fuzzy month year/12 # on average