Total hours in 2 weeks (14 days) is: 336h

Total hours in 2 week jam is:  312h (14h +288h + 10h) (day 1 + day 2 to 13 + day 14)

Hours slighted (336h - 312h)  = 24 hours

Jam length by hours (312/24) = 13 days

So by calculating the exact hours you would get that the jam length is only 13 days, though it lasts from the 6th to the 19th, which is 14 days. The solution (to get the exact right amount of time in hours) is to start the jam at 12:00 am on the first day and end the jam at 11:59 pm on the last day, adding exactly 24 hours to the time-frame and you would keep the date exactly the same by doing so. 

The argument of 'how many days till tomorrow' doesn't make sense to me at all because there are 0 days in-between today and tomorrow, but there are 2 days total. Though, I think you can make an argument for counting the hours and feeling like you don't have enough time. This is fair but the post title is misleading because the jam lasts for 14 days worth of dates, just not 14 days worth of hours (kind of confusing). An example is that from the 6th to the 9th there is 4 days; the 6th (1), the 7th (2), the 8th (3), and the 9th (4).  But, from the 6th starting from 10 am to the 9th ending at 10 am is only 3 days; 14h (1), 24h (2), 24h (3), 10h (4). Total hours: 72, total days: 3.

TL;DR: By counting the hours you get 13 days, but by counting the days you get 14 days. If you feel this is really important I would recommend directly messaging the jam host through social media (he links it on his page) about receiving a time extension for the jam. Also I think the post title is misleading, but that's semantics.