what is the reference of time

'Reference' referes to starting point of anything. for example: If a race starts at 10 am. and the first person finishes the race at 10:30, then it is said that the winner finished the race in 30 minutes. This 30 minutes is with reference to the starting time of the race (10 am. here).

Thus for any calculation we generally take some reference and then proceed further to make calculations simple.