Absolute Time starts counting from 0 when the TouchDesigner process starts, and (unlike the looping timeline) is always increasing. It will pause if the Power 0|1 button at the top of the UI is Off.
Absolute Time in TouchDesigner is expressed in seconds and in frames, where frame 1 corresponds to seconds = 0. If the global frame rate is 60, then frame 61 corresponds to 1 second, and so on. The global frame rate is most often locked set to be the rate of the display monitors on the computer running TouchDesigner, but in fact can be anything. See
cookRate in Project_Class.
The python class is absTime and its members are
absTime.seconds. See AbsTime_Class and absolute time.