I don't understand the question. The problem is that any number divided by 86400 except multiples of such will result in a floating point number.
Inclusive it is 92 days yeah. To do that you would use a time with the datetime you'd simply issue a modify on it and add one day.
I had your numbers backwards, I thought you were getting 92 and you were intending for 9100. Monetary should be dealt with using the money_format function.