add-log-time-format

add-log-time-format is a variable defined in `add-log.el'.
Its value is

add-log-iso8601-time-string

Documentation:
Function that defines the time format.
For example, `add-log-iso8601-time-string', which gives the
date in international ISO 8601 format,
and `current-time-string' are two valid values.

You can customize this variable.