Date::Calc and a little hand rolled code may help:

use strict;
use warnings;
use Date::Manip;
use Date::Calc qw(Add_Delta_DHMS);
use Benchmark qw(cmpthese);

Date_Init ("TZ=UT");

my @dates = ("2007-04-30 12:50:00", "2007-04-30 11:50:00");

print "Manip result: \n", join "\n", manip (), "\n";
print "Calc result: \n", join "\n", calc (), "\n";

cmpthese (-1,
    {
    manip => \&manip,
    calc => \&calc,
    });


sub manip {
    my @results;
    
    for (@dates) {
        my $date = $_;
        my $hour = (split /:/, $date)[0];

        die "Can't find hour" unless $hour =~ /\s(\d{2})$/;

        if ($1 >= 12){
            $date=DateCalc("$date","+24 hours"); ###Takes 2min out of 
+2:30min.
            $date=&UnixDate($date,"%Y-%m-%d");
        }
        
        push @results, $date;
    }
    
    return @results;
}


sub calc {
    my @results;
    
    for (@dates) {
        my $date = $_;
        my $hour = (split /:/, $date)[0];

        die "Can't find hour" unless $hour =~ /\s(\d{2})$/;

        if ($1 >= 12){
            my ($year,$month,$day, $hour,$min,$sec) =
                $date =~ /(\d{4})-(\d\d)-(\d\d) (\d\d):(\d\d):(\d\d)/;
            ($year,$month,$day, $hour,$min,$sec) = Add_Delta_DHMS (
                $year,$month,$day, $hour,$min,$sec, 1, 0, 0, 0
                );
            $date = "$year-$month-$day $hour:$min:$sec";
        }
        
        push @results, $date;
    }
    
    return @results;
}
[download]

Prints:

Manip result: 
2007-05-01
2007-04-30 11:50:00

Calc result: 
2007-5-1 12:50:0
2007-04-30 11:50:00

         Rate manip  calc
manip   662/s    --  -99%
calc  59112/s 8825%    --
[download]

A little tidying up to generate consistent output formating may be required, but that's likely not a big price to pay.

---

---

Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!

Titles consisting of a single word are discouraged, and in most cases are disallowed outright.

Read Where should I post X? if you're not absolutely sure you're posting in the right place.

Please read these before you post! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

Posts may use any of the Perl Monks Approved HTML tags:

a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr

You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)

	For:		Use:
	&		&
	<		&lt;
	>		&gt;
	[		&#91;
	]		&#93;

Link using PerlMonks shortcuts! What shortcuts can I use for linking?

See Writeup Formatting Tips and other pages linked from there for more info.