In the end you'll notice it just takes a couple of lines of code to do it - whatever you may write yourself will be longer and more error prone. See this example:

use Date::Manip;
$main::TZ = "CET";

my $ds = '2003/10/19'; # it's a sunday
my $d  = &ParseDate( $ds );
print "$ds = $d \n";

my $d2 = &DateCalc( $d,"+ 3hours 12minutes 6 seconds", \$err);  
print "$d2\n";

my $d3 = &DateCalc( $d,"+ 5 business days", \$err);  
print "$d3\n";
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Note how you can even use business days when doing date operations (that's why $d3 looks weird at first...). I think it's no point reinventing the wheel. :-)

On the other side, Date::Manip is big and slow, so if you have to compute thousands or millions of operations with it in a few seconds, it's not likely the way to go....