Stringent limits on the existence of Planck time from stellar interferometry

We present a method of directly testing whether time is `grainy' on scales of 1 Mpc by the Very Large Telescope Interferometer (the VLTI). Based upon the S Ser result, nonetheless, it is already possible to conclude that there is no first order departure from exactness in time over intervals ~ 4.2E-40 s (~10^4 times longer than the Planck time).