This gives the amount of time (in 1/10 seconds) that the world has been running. In actual fact, it is the number of server ticks that have passed multiplied by world.tick_lag. Therefore if the server sleeps (when no players are connected) this time is not counted. Also, if the server runs overtime during a tick (because procs take longer than tick_lag to finish) this still only counts as one tick. This value is therefore a measure of “game time” rather than real time.