No, not all days have 86400 seconds. (Update: Specifically, in places with Daylight Saving Time, some days are longer and some are shorter. The parent's code will result in the calculated date being off by a day when executed during certain hours of the year. )
If you were limited to core Perl, you'd could do something like the following:
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.
I think he means that if the 14 day interval happens to span the day that daylight-savings-time goes 'on' or 'off', then there will be an error (time difference) of one hour. Hypothetically, if you did this calculation around midnight of the day DST kicked in, you might get the wrong result...