According to the link below, 'You do not have to encode any of the characters manually'

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/rsprog/htm/rsp_prog_urlaccess_7kbq.asp

Unfortunately, this is not the case for...

Try the following!!!

=IIF(Fields!Margin.Value IS Nothing,"",Fields!completedDate.Value.toShortDateString)

Hope this helps!