make git ignore the timestamp embedded in PDFs

Hi all,

how can I make git ignore the time stamp(s) in a PDF. Two PDFs that differ only in these time stamps should be considered identical.

Here is an example:
,----
| > pdfinfo some.pdf
| Title:          R Graphics Output
| Creator:        R
| Producer:       R 2.15.1
| CreationDate:   Thu Jan 24 13:43:31 2013 <==  these entries
| ModDate:        Thu Jan 24 13:43:31 2013 <==  should be ignored
| Tagged:         no
| Pages:          1
| Encrypted:      no
| Page size:      504 x 504 pts
| File size:      54138 bytes
| Optimized:      no
| PDF version:    1.4
`----

What I tried is a filter:
,----[ ~/.gitconfig ]
| [filter "pdfresetdate"]
|         clean = pdfresetdate
`----

With this filter script:
,----[ pdfresetdate ]
| #!/bin/bash
|
| FILEASARG=true
| if [ "$#" == 0 ]; then
|     FILEASARG=false
| fi
|
| if $FILEASARG ; then
|     FILENAME="$1"
| else
|     FILENAME=`mktemp`
|     cat /dev/stdin > "${FILENAME}"
| fi
|
| TMPFILE=`mktemp`
| TMPFILE2=`mktemp`
|
| ## dump the pdf metadata to a file and replace the dates
| pdftk "$FILENAME" dump_data | sed -e '{N;s/Date\nInfoValue: D:.*/Date\nInfoValue: D:19790101072619/}' > "$TMPFILE"
|
| ## update the pdf metadata
| pdftk "$FILENAME" update_info "$TMPFILE" output "$TMPFILE2"
|
| ## overwrite the original pdf
| mv -f "$TMPFILE2" "$FILENAME"
|
| ## clean up
| rm -f "$TMPFILE"
| rm -f "$TMPFILE2"
| if [ -n $FILEASARG ] ; then
|     cat "$FILENAME"
| fi
`----

This 'works' as far as the committed pdf indeed has the date reset to my default value.

However, when I re-checkout the files, they are marked modified by git.

So, my question is: How can I make git *completely* ignore the embedded date in the PDF?

Many thanks in advance for any help!

Regards, Andreas

PS: I had posted this question (without much success) here: http://stackoverflow.com/questions/16058187/make-git-ignore-the-date-in-pdf-files and with no answer on the git-users mailing list: https://groups.google.com/forum/#!topic/git-users/KqtecNa3cOc