DateDiff() is usually used to perform date difference calculations. But for simple difference calculations you can subtract dates from each other, like this:
<cfset mydate=CreateDate(2006,8,17)>
<cfoutput>#Now()-mydate# days since #DateFormat(mydate)#</cfoutput>
The returned number will likely not be an integer, and will contain date fractions too, so you may want to use Int() to round the number to just the integer portion:
<cfset mydate=CreateDate(2006,8,17)>
<cfoutput>#Int(Now()-mydate)#</cfoutput>
(Now() - 1) === > Yesterday
(Now() + 1) === > Tomorrow
--- Ben