Kevin,
If I may step in, I'll try to explain without waxing techno-theoretical. First, where is this field that contains this expression and what is it's purpose? Is it a report result field?
Most importantly, are you doing any summary calculations on it? This is where the Null result (you can take my example and replace '0' with Null to do this) is key. If it's just a display issue, and if zero works for you, then leave it as is. However, if there are caluclations, then it's up to you to decide: Is the true value of the result zero, or is it simply not applicable since there were no hours, and this result will poison any subsequent calcs on this field. If it's Null, then summary calcs, such as Avg & Count, will ignore it, which is mathematically the right thing to do (I can hear Wilford Brimley saying that!).
As Michael implied, in a somewhat theoretical way, is that if you divide by zero, the result is simply 'not applicable' and should be excluded from any further summary calculations--ie the true mathematical result is not zero. Again, if you just want to see a zero instead of blank, leave it, else replace with null--not the highest value the processor can create since this will *really* increase the confusion.
--Jim