Formula for # of days since today, if blank or if cell filled in. Want in number format?

Please see attached sheet...

Regarding Q column....

If the J column is blank, that means we haven’t received it back yet. I want to make a formula in Q, saying if this is blank, then work out how many days before today the repairer has had it (as a minus number) ie, received 09/07/12, but not here yet (imagine todays date is 12/07) so cell is blank – so it says something like ‘-3’ – also this is to be in working days only. If it ISN’T blank (we fill it in when we receive the goods back), I want it to say how many days since the ‘recieved date in column h’ til the date that will be entered in the j column. Ie. If received 09/07, and fixed 12/07, I want it to say 3 (as in a +, not a neg). Then I can use these numbers to conditionally format and create a traffic light system to quickly show what is overdue and pending etc. (But I know how to conditionally format, so the question is just on the formulae required)

Any help appreciated.
Thank you!