The computer has several kinds of time
Wallclock time is actual elapsed time
On a timesharing system, this is rarely the amount of time the process actually spent working
It shares the processor with the OS and with other processes
Of the wallclock time, some was spent executing instructions in the process's program
For example, copying data around or doing tests
This is the user time
Some time was spent by the OS executing OS instructions at the program's request
For example, fetching mtimes, performing I/O, and allocating memory
This is the system time
user time + system time = CPU time <= wallclock time