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### 8.5 Date Arithmetic

The commands described in this section perform various conversions and calculations involving date forms (see Date Forms). They use the `t` (for time/date) prefix key followed by shifted letters.

The simplest date arithmetic is done using the regular `+` and `-` commands. In particular, adding a number to a date form advances the date form by a certain number of days; adding an HMS form to a date form advances the date by a certain amount of time; and subtracting two date forms produces a difference measured in days. The commands described here provide additional, more specialized operations on dates.

Many of these commands accept a numeric prefix argument; if you give plain `C-u` as the prefix, these commands will instead take the additional argument from the top of the stack.