Here is a solution from within PM.
Select/highlight all the pictures you want the date changed to. Then open your ITPC Stationary pad( command I for the mac), then in that window go to the "date" , click that box at left, then to the right select "date" and set the date you want, then at bottom of ITPC Stationary Pad click on button "apply stationary to selected."
Done and the ITPC of selected picture(s) is set to the desired date.
See attached
So other ITPC info is not changed or deleted make sure you uncheck all the boxes of ITPC Stationary Pad. If you do not want any other changes to your ITPC then the easy way is you can click on "clear" at bottom left of Stationary Pad so all boxes are unchecked. Do this before the above steps.
Food for thought
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