It may have gotten reset by the Control Panel's Regional Settings control.
Too see what Excel is using by default, press ALT F11 to open the VBA
editor, press CTRL G to display the Immediate window, and type the following
and press ENTER.
?Application.International(xlDateSeparator)
This will display the character (typically '/' in US-English) that Excel
uses to separate date elements. If it is not the same, close Excel, go to
the Window Start menu, choose Control Panel, and select "Regional Settings"
or "International Settings" or some such thing (its name varies between
versions of Windows). In that dialog, ensure the proper date separator is
selected.
--
Cordially,
Chip Pearson
Microsoft MVP - Excel, 10 Years
Pearson Software Consulting
www.cpearson.com
(email on the web site)