How to install:

1. download
2. unpack in World of Warcraft\Interface\Addons\
3. check that it's activated in Addons button

Clock command help:
Use /clock <command> to perform the following commands:

help     : displays this message.
status   : displays status information about the current option settings.
freeze   : freezes the Clock so that it can't be dragged.
unfreeze : unfreezes the Clock so that it can be dragged.
reset    : resets the Clock to its default position.
24-hour  : displays the time in 24-hour format.
12-hour  : displays the time in 12-hour format.

Anything else is interpreted as the offset from server time that is applied to the time displayed.
Supported formats include -:30, +11, 5:30, 0, etc.

For example: /clock +2 will adjust the displayed time to two hours later than server time.