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:

use POSIX       qw( strftime );
use Time::Local qw( timegm_nocheck );

# Get date.
my ($y, $m, $d) = (localtime())[5, 4, 3];
$m += 1;
$y += 1900;

# Add to date.
$d += 14;

# Canonize date.
($y, $m, $d) = (gmtime(timegm_nocheck(0,0,0, $d, $m-1, $y)))[5, 4, 3];
$m += 1;
$y += 1900;

print(strftime("%x", 0,0,0, $d, $m-1, $y-1900), "\n");
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Note: I like keeping $y, $m and $d as a human readable numbers. You could shorten the above by leaving $m 0-based and $y 1900-based.

Note: I don't recommend this approach. Take a minute to install one of the date modules.