You could experiment with producing a PDF as a form with a field for the date, which your client could complete. Not sure what the quality would be for printing. Alternatively if it's for say a magazine ad, the media owners might be able to do it for you, using Acrobat Pro. Might be worth having a word with them.
Re the JPG, there are free and low cost (Photoshop Elements) applications which your client could use to open a JPG, add a date, and re-save it as a JPG for uploading to his/her site.
i will try making it a form
Don't forget to Reader Enable the form, otherwise your client won't be able to save the date!