Do I have to take timestamp of sending original report always as 0.00000
secs?
No, just work out the difference in time between the 2 packets
involved in the calculation - it really isn't difficult. Experiment
with the different views in 'View | Time DisplayFormat' and work out
what they mean.
You could even select the first of the 2 RTCP reports used in a
calculation and choose 'Edit | Set Time Reference (toggle)'. The
timestamp shown for the later frame will then show the time difference
you need.