Residential Rental Days -- How are 15 days calculated?

[Matt H.]

Hello -- I am interested in short-term renting my house for a bit at the end of this year and ran across the 15 day rule for excluding the rental income from taxes. I wanted to see if anyone knew the specifics for how those 15 days are calculated? As things stand now I have 13 nights booked with a check-in time of 4pm and check-out time of 11am. Does anyone know if both the day of check-in and check-out count as full days for tax purposes? If so, even though I only have 13 nights booked am I at risk of exceeding 14 days? Thanks!!