If you used exactly what you posted, you'd get a syntax error so I would guess that what you really have is:
Set Field [Invoice::INPostedDate; "2/01/2013...$PostedDate"]
IF that's what you have, the variable is enclosed in your quotes and that causes fileMaker to see $PostedDate as literal text, not a variable containing a value. Use:
Set Field [Invoice::INPostedDate; "2/01/2013..." & $PostedDate]
I knew that.
Thanks for the help.