I need a date/time stamp for cell entry

I have a speradsheet that I want to use to log date and time information at the time of entry.

Cell A1 = Item number (6 or 7 digit number)
Cell B1 = Initials of person entering data
Cell C1 = Assigned area (drop-down list to select)
Cell D1 = Date and time of entry (to be automatically entered with formula. Not to fill in unless all three of the other cells are filled out - A1 w/#, B1 w/text, C1 w/list item)

Thanks Much!!

Works Great.