How can I calculate how far through the day the prime meridian is of different planets I'm trying to create a widget that shows the 'time' on different planets. It will show how far through the day/night cycle (as a percentage) a point on the planet is. It has been easy to scale down the cycle from 24 hr to eg ~10hr day of Jupiter however I am struggling to work out how I can 'set' the time. 

For Mars this has been straight forward, using the comparison of MTC and UTC of when the Curiosity rover landed but I can't find similar data for other planets to anchor time from earth to the other planets. 

Is there a data base for this kind of information?
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.