Great - there’s the magic secret… according to this the epoch is new year’s day 1900, and each number is a day since.
You could use the date & time shift node - but that seems like a pain in the ass. Here’s a workflow. Perhaps you could export the dates in a different format (like an YYYYMMDD string or milliseconds since epoch or …) for which there are nodes that do this conversion specifically.
plunket excel number to date.knwf (6.8 KB)