How can I calculate how far through the day the prime meridian is of different planets
NASA's Navigation Ancillary Facility (NAIF) publishes planetary constants kernels (PCK) which are basically text files containing pole orientations for the largest known bodies. 

PCKs includes the parametric orientation of their prime meridian in agremeent with IAU standards (meaning datum is J2000.0).

The latest PCK dates from 2011 and is actively used by most operational interplanetary flight projects across the world.

Using the NAIF SPICE library (available in Fortran, C, Matlab and IDL) you can load this kernel and read the orientation of the prime meridian for your body and date of interest. 

Time offset between bodies can be reconstructed as angular offset between meridians. But you may find some time-related functions within SPICE which can make your task easier.