BASH: Keep original mod date of a file and 'date' it back to same file?

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http://www.linuxquestions.org – I would really like to preserve a file's original modified date and pass it back to the file as the same attribute after a script has worked on it. I get a lot of JPEG files from different places on the Net which I either turn around and upload or burn to disk, and having the "original" date of either download or last mod in a graphics app would be for me, in the long run, a lot more helpful when deciding, for instance, which files to "recycle" or pass on backing up more than once. I've tried doing this on my own every now and then. Where I run into problems is (HowTos)