Get time before of fft block

Hi,
I’m a new user of gnuradio.
I want to use this software to analyze a traffic on wifi channels.I
have a
usrp2 hardware and I use the program file usrp2_fft.py to capture and
then
save into a file the result frequency and amplitude. The problem is :
how
can I get the time data sampling (in the simpler I think) that occurs
prior
to FFT ?
PS: sorry for my english and thanks.