Wrap the d in double quotes, not 2 single.
DateDiff(interval, date1, date2 [, firstdayofweek] [, firstweekofyear] )
Example:
=DateDiff("d", Now(), [Orders].[ReceiveBefore])-10)
http://office.microsoft.com/en-gb/access-help/datediff-function-HA001228811.aspx