Phase lag from fft, is it the actual lag?

Hello, So basically I am applying a sinusoidal forcing function to a system. Which means, since my forcing function is a sin function, it already has a pi/2 phase. Now I get data of the output response that I am suppose to extract the phase lag from. I use fft on the output response and I get pi/2, now this where I get confused. Since the original forcing function has a pi/2 phase, and now my lag says it is also pi/2, does it mean they are in phase? is the actual lag = 0? Or, is my output response pi/2 delayed from the input? so they are out of phase? The reason I am confused is because my lag from fft and my original phase are both pi/2. Can someone please direct me to the right place... Thanks so much in advance...

Answer 1

[Matt J]

Possibly the lag is a multiple of 2*pi ?