  The radio as well as the high energy emission mechanism in pulsars is yet not
understood properly. A multi-wavelength study is likely to help in better
understanding of such processes. The first Indian space-based observatory,
ASTROSAT, has five instruments aboard, which cover the electromagnetic spectrum
from infra-red (1300 $\AA$) to hard X-ray (380 KeV). Cadmium Zinc Telluride
Imager (CZTI), one of the five instruments is a hard X-ray telescope functional
over an energy range of 20-380 KeV. We aim to estimate the timing offset
introduced in the data acquisition pipeline of the instrument, which will help
in time alignment of high energy time series with those from two other
ground-based observatories, viz. the Giant Meterwave Radio Telescope (GMRT) and
the Ooty Radio Telescope (ORT). PSR B0531+21 is a well-studied pulsar with
nearly aligned radio and hard X-ray pulse profiles. We use simultaneous
observations of this pulsar with the ASTROSAT, the ORT and the GMRT. The pulsar
was especially observed using the ORT with almost daily cadence to obtain good
timing solutions. We also supplement the ORT data with archival FERMI data for
estimation of timing noise. The timing offset of ASTROSAT instruments was
estimated from fits to arrival time data at the ASTROSAT and the radio
observatories. We estimate the offset between the GMRT and the ASTROSAT-CZTI to
be -4716 $\pm$ 50 $\mu s$. The corresponding offset with the ORT was -29639
$\pm$ 50 $\mu s$. The offsets between the GMRT and Fermi-LAT -5368 $\pm$ 56
$\mu s$. (Abridged)
