How to get the precise time instant of a strobed waveform

Hi All,

Assume in the waveform there are N1+N2 strobe points, the strobe time is defined by me which is T1 and T2.

How I can get the last strobe point time in the waveform N1 or 1st strobe point time in the waveform N2, e..g, by calculator?

 Thanks