Hourglass
Hourglass timing is a game clock time setting, where both players start with identical absolute time, and the amount one player spends on thinking is added to the other player's time pool, so the total remaining amount of time (sum of each player's remaining amount of time) stays constant. For practicability, hourglass timing systems may have a certain delay (a few seconds usually) on the beginning of each move before the "sand starts to fall down". Otherwise it might be too easy to force a time win by playing nonsensical moves as quickly as possible.
Available on Kaya Go Server.