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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.