You need to 'unprotect' the cells before protecting the sheet. e.g. as follows. You could include the date cell (as here B1) in the list of unprotected cells:
Code:
With Sheets("MainDataEntryForm")
.Unprotect Password:=""
.Range("B1,B2,B7,B10").Locked = False
.Protect Password:="", UserInterfaceOnly:=True
.Range("B1") = Date
End With