Hi Beardking,
To my knowledge, it is not possible to look at 'previous' sheet just by using functions. But what you can do is to write code that will 'find out the current spreadsheet' and then look for 'previous' spreadsheet and get the date from there.
If I were you, I would do this manually as this is minor work (please correct me if I am wrong).
Regards,
Chaitanya