Excel has nothing to with this – the error is thrown from your OS, which probably is unhappy about certain characters in the textual representation of the date (namely, the slash).
StyleMaster::Factory & " " & " ShipSchedule" & Substitute ( Get ( CurrentDate ) ; "/" ; "_" )
My guess is the "Date" would look something like this "3/29/2016", file names don't like the "/" symbol in them. You will need to change the date format in your calculation
Year(Get(CurrentDate)) [will produce 2016] & Day(Get(CurrentDate)) [will produce 29 ]
End result is 201629
* You may need to switch "Year" and "Get" in the calculation, I can't remember which way Filemaker likes that calculation set up. Then you can add hyphens where needed and change the date order however you choose.
edit: erolst has the right idea.
The "/" was in fact the culprit! Thanks much for the assistance.