Converting date serial number to date

How does one convert a date serial number copied from a file of another spread sheet (Quatro Pro) into a formatted date? Currently, the cell is not formatted but contains only the date serial number. There are many unformatted cells on the spread sheet which need conversion.

I am new to Excel, so I overlooked the obvious. To convert a date serial number in an unformatted cell, simply click on the column head, format cell, date, and the type of date format desired.