Negative value of latency

Hello all,

I've done some measurement on some system using Linux about latency. Measurement is based on periodic activation of processes.

Latency = (Current Activation TIme - Previous Activation TIme) - Period Of Activation

I would like to know if this formula is valid. If not, please correct me. Another issue is the results I've obtained contains negative latency value. I've looked around the web but unfortunately can't find the explanation of the existence of the negative value. I really would like to know what causes the negative value. It would be very appreciated if anyone can explain about this. Thanks.