What is meant by start time in powergui fft tool,How does start time and no.of cycles is related to the THD?

i see the variation in THD with change in start time and no. of cycles. can any one please help me to get rid of this confusion regarding such variation? How start time and no. of cycles relates with THD ur suggestions are always welcome. (/i am carrying out project on mitigation of harmonics/).

[Ahmed Shaaban]

I'm seeking the answer for the same question. Hopefully someone can help us.