PDF to TIFF hash - Avoid date/time reference

Hi,
    I'm using Ghostscript to extract pages' images from a PDF using a
command like:

gs -dQUIET -dNOPAUSE -dBATCH -dSAFER -dFirstPage=1 -dLastPage=1
-dBandHeight=100 -dNOPAUSE -sDEVICE=tiff12nc -r200
-dNumRenderingThreads=2 -dBandBufferSpace=500000000
-dBufferSpace=750000000 -sBandListStorage=memory -sOutputFile=page1.tiff
input.pdf

For caching purposes I need that the hash of each tiff must be the same
if it's produced from the same page. Unfortunately, I noticed that a
date/time reference is added at the "beginning" (prolog?) of each file.
That's the only thing that alters the hash: if you produce images in the
same second, the hash is the same.

So, is it possible to avoid to add that time reference?
If it's not possible, is there a way to test if two tiffs have the same
"appearance", without considering the metadata?

I suppose the last chance is to change the source code and recompile it :)

Thanks,
    Giacomo Boccardo