I found this on one of our Info pages.

Question

What is the clock at the bottom of the page in a game and how does it work?
Answer
The clock displays the total time that it has been yours and your opponent's turn since the game began. It is a running total. This helps to indicate how long it takes you to make a move. The numbers represent Days, Hours & Minutes. The clock at a player's name (also in multi player games) shows how much of the total time for the game it has been that player's turn to play. This means that if you move immediately after it becomes your turn your clock will only advance slowly, because the clock is stopped when it is not your turn. Let me try to clarify with an example. Players A and B both make 1 move a day, and B makes his move 5 minutes after A has made his move. This means that B's clock only advances 5 minutes per day while A's clock advances 23 hours and 55 minutes a day.

:

Underneath each game is the clock that denotes where you stand on time with that particular game. It looks like this:

White: xxxxxxx (1592 Friendly) [Clock: 4:02:10]
Black: gggggg (1675 Friendly) [Clock: 51:21:09]

The clock itself = equals days:hours:minutes

So this [Clock: 51:21:09] means gggggg has used 51 days, 21 hours and 9 minutes total in that specific game.