Calculating the day number of MESSENGER MDIS images:

MDIS EDRs have the following naming convention, copied from 
http://pdsimg.jpl.nasa.gov/data/messenger/MDIS/DOCUMENT/MDISEDRSIS.PDF:

"Format: 'xc##########f.IMG'
X = E for EDR
c = W or N for Wide or Narrow Angle camera
########## = Mission Elapsed Time (MET) counter taken
from the image header (and same as original compressed
filename from SSR).NOTE: this a spacecraft clock seconds
counter, and the value in the filename corresponds to the
LAST second of the exposure.
f = Filter wheel position (A,B,C,D,E,F,G,H,I,J,K,L,U) for
the WAC. (See Table 5, Appendix B) It is M for the NAC,
which has no filter wheel. It will be U if the position
is unknown." 

Inconveniently, EDRs are arranged by ordinal day number and named using 
MET. However, day number can be calculated from MET using the following 
formula: floor([MET in product ID - MET at beginning of day 88 (i.e., 
209843980 s)]/(86400 s/day) + 88). For example, the remarkable image 
EN0220591163M has day number 212. 

This formula is approximate because 1) the rate of time increase of the 
spacecraft clock is piecewise linear, not linear; 2) I haven't yet been 
able to figure out exactly when the spacecraft clock started (nominally 
6:00:02 AM UT August 3, 2004). 

Will Vaughan