=========
EXAMPLE
———
=Today()-1

=DATE(2011,4,15)-1

=========
REF
———
http://www.excelforum.com/excel-worksheet-functions/570878-today-minus-1-day.html 

http://www.cpearson.com/excel/datearith.htm

—
http://www.anysitesupport.com/excel-date-minus-1/