How do the time of the occurence of p-waves help you know the time the earthquake occurs?

Question

How do the time of the occurence of p-waves help you know the time the earthquake occurs?

Answer 1

You won't know the exact time because the speed of p waves changes as they travel through different materials.  However, you can get a fairly accurate time.  You must know the speed of the p waves (speeds vary between 1.5 and 8 kilometers per second),the distance you are from the epicenter and the time p waves were first felt at your location.

Then it is simple math from there.

Example:

If you are 300km away from the epicenter and you first feel p waves at 7 AM and we assume that the p wave speed averages 3 km per second, then the earthquake occured at  6:58:20 AM.