Your question is a bit more complicated than you might think,
It depends on the format of the date in the spreadheets you get.
If the input date cellls are formatted as strings, your best option is to fashion the strings so that you can construct a date object out of it in app script.
Since you apparently want a timestamp value rather than just a date (format DateTime), then the data should already be a date object in the cells.
Either way, you need to start out with a date object. Retrieve the cell(s) with getValue() NOT getDisplayValue(), and then stringify it to whatever format you want with Utilities.formatDate().