How to prevent a cell from calculating formula until all data is entered

[McFatty]

I am trying to do something that I imagine isn't that hard, but I'm not an excel super user, just a novice with google. I took over a spreadsheet that tracks our generators run time. It is simply just subtracting the time on the run meter for the current month, from the prior month. Example: =+B58-C57 (B being the start time of current month, C being end time of previous month) The problem is since the cell for the current month is blank (the B cell), it is subtracting the end time from a 0 value, giving me a negative number for that calculation. This throws off the total. It seems I may need to use an IF or THEN function, or maybe there is an easier way. Any help would be appreciated.


My IT's security doesn't seem to want me to use any tools to upload attachments so you may have to work blind on this one/

 

[alansidman]

=if(B58="","",B58-C57)

Try this.

 

[McFatty]

You sir are fast, smart, and correct. That worked perfectly. Much appreciated, I've been trying off and on for 2 days to figure this out and you did it, with no picture to reference, 6 minutes after my post. Thank you!