  The LISA mission is a space interferometer aiming at the detection of
gravitational waves in the [$10^{-4}$,$10^{-1}$] Hz frequency band. In order to
reach the gravitational wave detection level, a Time Delay Interferometry (TDI)
method must be applied to get rid of (most of) the laser frequency noise and
optical bench noise. This TDI analysis is carried out in terms of the
coordinate time corresponding to the Barycentric Coordinate Reference System
(BCRS), TCB, whereas the data at each of the three LISA stations is recorded in
terms of each station proper time. We provide here the required proper time
versus BCRS time transformation. We show that the difference in rate of station
proper time versus TCB is of the order of $5 10^{-8}$. The difference between
station proper times and TCB exhibits an oscillatory trend with a maximum
amplitude of about $10^{-3}$ s.
