  The AstroSat mission carries several high-energy detectors meant for fast
timing studies of cosmic sources. In order to carry out high precision
multi-wavelength timing studies, it is essential to calibrate the absolute time
stamps of these instruments to the best possible accuracy. We present here the
absolute time calibration of the AstroSat LAXPC instrument, utilising the
broadband electromagnetic emission from the Crab Pulsar to cross calibrate
against Fermi-LAT and ground based radio observatories Giant Metrewave Radio
Telescope (GMRT) and the Ooty Radio Telescope (ORT). Using the techniques of
pulsar timing, we determine the fixed timing offsets of LAXPC with respect to
these different instruments and also compare the offsets with those of another
AstroSat instrument, CZTI.
