Frequency is the reciprocal of time. For example, if you measure your square wave as 16.66...milliseconds, you have a 60 Hz. frequency.

Now, dependent on whether your milliseconds are floating point (many decimal places), you can measure frequency with concomitant resolution. That is, if time is an integer and you only have choices of 16 or 17 milliseconds, the frequency comes out 62 Hz. or 58 Hz. respectively while the actual waveform is 60 Hz..

Jim