Interesting Excel challenge

Hello,

I have an interesting Excel 2007 problem. Well, it's a problem to me since I can't solve it. Hopefully my explanation is clear to you:

I am doing some simple calculations in excel where I have to calculate the amount of days between the columns 'sent out to approve documents' and 'approved documents'. I would like to track how much time it takes for each document (row) from the date I sent the doc out to the date it was actually approved. In case it's not approved yet BUT sent out before I am counting from the day I sent it out to today's date (now() ).
I don't want to show a value in either of the two columns and would like to show a value in the calculating cell (# of days) for all the possible scenarios (sent out but not approved yet / sent out and approved).

How can I do this? I got far, but not there yet. This is what I did:
I use 'IF(0, Then, NOW())' for the cells where no date is included yet and use ';;;' to hide this value. Furthermore, in the calculating cell (#days) I put '0;-0;;@' in the format to hide zeros. The calc is: 'cellY-cellX'.
The problem is that when I hide the cells with 'IF(0, Then, NOW())' and want to put the actual date in when it's approved or sent out, it hides (because of the ;;.

There must be a solution to this and it would be awesome if someone could help me!

Thanks,
Nick