SessionTime[ ] gives the total number of seconds of real time that have elapsed since the beginning of your Mathematica session.
SessionTime starts counting time as soon as your operating system considers your Mathematica process to be executing.
SessionTime is accurate only down to a granularity of at least $TimeUnit seconds.
See The Mathematica Book: Section 2.13.3.
See also: TimeUsed, AbsoluteTime, Date.