Working with duration and numbers

Question

I have rental apartments and I am building a spreadsheet to show how many months each tenant has occupied his apartement and how much rent he paid in total.
Column A shows the move-in date
Column B shows the move-out date
Column C shows the number of months : (B-A)/30
Column D shows the monthly rent amount
Column E shows the total amount paid for the duration of the tenancy : C*D
Everything works out fine except for the amount shown in Column E.  Here is an example :
Column A (move-in date) : 2011-08-01
Column B (move-out date)  : 2012-05-31
Column C (total months) : 10
Column D (monthly rent)  : $500.00
Column E (total paid) : $5066.67
Column E should show $5000.00. 
I know that Column C is the problem.  Although the number shown in the cell is 10, the real number in the background looks like this : 10:3:12:00.
How do I get Numbers to multiply the amount in Column D by the number that appears in the cell (number 10)?

Answer 1

You are working with three types of value here: Date and Time values, Durations, and numbers.
When you subtract a Date and Time value from another, the result is a Duration. If both D&T values were entered as dates, the time part of those values will have been set to 00:00:00 (midnight at the beginning of that day), and the duration value will be an even umber of days.
The equation in column C assumes that all months have 30 days. Makes things neat, but unfortunately doesn't jibe with reality as defined by the calendar makers. B-A in your example returns 304d (304 days)
The result (5066.666666...) is correct for a rate of 304 days @ 500/30 days
The easiest fudge factor to make the number in C work out correctly is to convert the value in C to a number, then ROUND the number to the nearest whole number. For long periods, this will differ from the number of whole months, and it will not correctly calculate the results where the tenant moves in (or out) mid-month. If that's an issue, we can look for a more detailed solution (or check in the files to find a previous one, assuming I kept a copy).
Replace the formula in C with: =ROUND(DUR2DAYS(B3-B2)/30,0)
Regards,
Barry