As indicated above, the time arithmetic functions are accessed in the
timer device structure as if they were a routine library. To use them, you
create an iorequest block and open the timer.  in the iorequest block is a
pointer to the device's base address. This address is needed to access
each routine as an offset - for example, _LVOAddTime, _LVOSubTime,
_LVOCmpTime - from that base address.

     timer_arithmetic.c 

 why use time arithmetic?