not all days are the same length.
Are you talking about a Leap_second? If Wikipedia is to believed, there have been 23 leap seconds since 1972. Since the OP is only trying to find what date is 14 days into the future, sweetblood's method is only going to have a problem on one second of a day with a leap second. That is, 23 out of the last 1093273318 seconds, as I write this, or one in 47.5 million (y'know, approximately).
I routinely say $ONE_DAY = 24 * 60 * 60. Maybe I'm sloppy, or I just haven't ever written an application where it matters. I suspect if I ever had a problem, I didn't notice or didn't care.