https://github.com/SciTools/iris/blob/5b42f47e71fbeb7861a9df59c8bd8c0be9a340e3/lib/iris/fileformats/pp_load_rules.py#L551 My code just fails as it tries to read in a file with a calendar type 'proleptic_gregorian' `cf_units` has several calendars: ``` CALENDAR_STANDARD = "standard" CALENDAR_GREGORIAN = "gregorian" CALENDAR_PROLEPTIC_GREGORIAN = "proleptic_gregorian" CALENDAR_NO_LEAP = "noleap" CALENDAR_JULIAN = "julian" CALENDAR_ALL_LEAP = "all_leap" CALENDAR_365_DAY = "365_day" CALENDAR_366_DAY = "366_day" CALENDAR_360_DAY = "360_day" ``` I just have this issue when I try to load an orography file, which has set the year to 0.