The time() function you are using returns a time in Epoch seconds (for Unix systems the number of seconds since the first of January 1970). So to work with another date you only need first to convert it to Epoch seconds. Time::Local will do this for you-

use Time::Local;
use POSIX qw[strftime];

my $other_time = timelocal($sec,$min,$hours,$mday,$mon,$year);

my $days = 30;
my $otherExpiration = strftime('%Y%m%d', localtime($other_time + 60 * 
+60 * 24 *
+ $days));
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Don't forget that months are counted from 0-11. Depending on the accuracy required you can skip parsing the time and just call timelocal() with a fixed time (say midday) and the parsed date- this is OK for an emailed reminder but not so great for a battery replacement on a pacemaker :) If the date data you are working with has differing formats or accuracy is critical then your safest approach would be to use Date::Manip or Date::Calc.

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Until you've lost your reputation, you never realize what a burden it was or what freedom really is. -Margaret Mitchell