Numbers Time Sheet Doesn't Work

I've been trying for 2 days to put together a simple spreadsheet in numbers to track my hours at one of my jobs. I am having several issues. First, I need the "hours" column (D2-D17) to record the time in 1/4 hour increments rather than hours and minutes (7.25h rather than 7h 15min). Second, I can only get the "Total Hours" column (D18) to correctly total up hours worked if ALL the rows above it (D2-D17) contain time values in them. This will rarely, if ever happen. If I haven't worked 1 or more days in the pay period and the row is blank I get the following error message in D18 "values being summed by SUM must be either all duration or all numbers, unless one of the values is a date" (see below). Cells B2-B17 and C2-C17 are formatted as "date and time" with "none" selected under date and "19:08" selected under time. Cells D2-D17 are formatted as "duration" cells with hours and minutes selected and format option set to "none" the formula in these cells is: SUM(C2−B2). Cell D18 is formatted as "automatic" with the formula "SUM(D2:D17)". I have tried many different format/formula combinations in the above cells but cant get anything to work. Thanks in advance for any help you can give. If it makes any difference, I've been working on my iPhone 6+.