hannibal01:
I know how to display real time but don't know how to measure it.
You mean compare two times and work out the difference? You must convert each time into a number of seconds after a fixed point in time, for example midnight last night, or midnight on 1st Jan. Then you can subtract one result from the other, and finally convert that number of seconds into hh:mm:ss again.
Do you need to measure to tenths or hundredths of a second? An RTC will not do that.