Here are some sample diary entries, illustrating different ways of formatting a date. The examples all show dates in American order (month, day, year), but Calendar mode supports European order (day, month, year) and ISO order (year, month, day) as options.
4/20/12 Switch-over to new tabulation system apr. 25 Start tabulating annual results 4/30 Results for April are due */25 Monthly cycle finishes Friday Don't leave without backing up files
The first entry appears only once, on April 20, 2012. The second and third appear every year on the specified dates, and the fourth uses a wildcard (asterisk) for the month, so it appears on the 25th of every month. The final entry appears every week on Friday.
You can use just numbers to express a date, as in ‘month/day’ or ‘month/day/year’. This must be followed by a nondigit. In the date itself, month and day are numbers of one or two digits. The optional year is also a number, and may be abbreviated to the last two digits; that is, you can use ‘11/12/2012’ or ‘11/12/12’.
Dates can also have the form ‘monthname day’ or ‘monthname day, year’, where the month’s name can be spelled in full or abbreviated (with or without a period). The preferred abbreviations for month and day names can be set using the variables calendar-abbrev-length, calendar-month-abbrev-array, and calendar-day-abbrev-array. The default is to use the first three letters of a name as its abbreviation. Case is not significant.
A date may be generic; that is, partially unspecified. Then the entry applies to all dates that match the specification. If the date does not contain a year, it is generic and applies to any year. Alternatively, month, day, or year can be ‘*’; this matches any month, day, or year, respectively. Thus, a diary entry ‘3/*/*’ matches any day in March of any year; so does ‘march *’.
If you prefer the European style of writing dates (in which the day comes before the month), or the ISO style (in which the order is year, month, day), type M-x calendar-set-date-style while in the calendar, or customize the variable calendar-date-style. This affects how diary dates are interpreted, date display, and the order in which some commands expect their arguments to be given.
You can use the name of a day of the week as a generic date which applies to any date falling on that day of the week. You can abbreviate the day of the week as described above, or spell it in full; case is not significant.