How to time contractions?

G
Feel kinda silly asking but when you time your contractions do you time them by the start of each new one or by the end of each one? I have baby center app and the way they count it is time in between each end and start of a new one, if that makes sense. Ex. If I had one that lasted 5:04-5:05 and the next one 5:10-5:11 it means they are 5 or 6 mins apart? Clearly I'm over thinking this....pregnancy brain smh