If you have the according fonts on your machine, you can change the date using Acrobat (Pro).
If you don't have the font on your machine, and it is acceptable from the appearance, you might also place a form field on top of that date, select the suitable font, make the background white (or the color it has to match), enter the date and use that. You might consider flattening the page, so that there are no more fields (but do not save this document unless you have made a backup copy of your original file ).
Hope this can help.
Max Wyss.