Display last modified time (datetime) of item.

== Example ==

$Item->mod_time( $format = '', $useGM = false );

== Parameters ==

* Name: format
* Default: (no space)
* Usage: Date/time format (Leave empty to use locale default date format). See Date Format for more information about date and time format.
* Name: useGM
* Default: false
* Usage: True if you want GMT.

== Return value ==

This tag displays content only, it does not have a return value.