Objective: Determine if a given day is an occurrence of a bi-weekly event.

Approach: Given the complexities of dates, opted to use a date module, hence DateTime::Set.

Code:

#!/usr/bin/perl -w

use strict;
use diagnostics;
use DateTime;
use DateTime::Set;
use Test::More;

my $biweekly = DateTime::Set->from_recurrence(
        'recurrence' => sub {
                return $_[0]->truncate('to' => 'day')->add('days' => 1
+4)
        },
        'start' => DateTime->today()
);

print 'FIRST 5 ELEMENTS OF SET', $/;
my $iterator = $biweekly->iterator;
for (0..4) {
        my $date = $iterator->next;
        print $date->datetime, $/;
}
print $/;

print 'TESTING whether Date is in set...', $/;
my $date = DateTime->today();
is($biweekly->contains($date), 1, $date->ymd . ' *IS* in the bi-weekly
+ set');
$date->add('days' => 7);
is($biweekly->contains($date), 0, $date->ymd . ' is *NOT* the bi-weekl
+y set');
$date->add('days' => 7);
is($biweekly->contains($date), 1, $date->ymd . ' *IS* the bi-weekly se
+t');
$date->add('days' => 7);
is($biweekly->contains($date), 0, $date->ymd . ' is *NOT* the bi-weekl
+y set');
$date->add('days' => 7);
is($biweekly->contains($date), 1, $date->ymd . ' *IS* the bi-weekly se
+t');
# ...

done_testing();
[download]

Output:

FIRST 5 ELEMENTS OF SET
2013-05-12T00:00:00
2013-05-26T00:00:00
2013-06-09T00:00:00
2013-06-23T00:00:00
2013-07-07T00:00:00

TESTING whether Date is in set...
ok 1 - 2013-05-12 *IS* in the bi-weekly set
not ok 2 - 2013-05-19 is *NOT* the bi-weekly set
#   Failed test '2013-05-19 is *NOT* the bi-weekly set'
#   at ./off-friday line 28.
#          got: '1'
#     expected: '0'
ok 3 - 2013-05-26 *IS* the bi-weekly set
not ok 4 - 2013-06-02 is *NOT* the bi-weekly set
#   Failed test '2013-06-02 is *NOT* the bi-weekly set'
#   at ./off-friday line 32.
#          got: '1'
#     expected: '0'
ok 5 - 2013-06-09 *IS* the bi-weekly set
1..5
# Looks like you failed 2 tests of 5.
[download]

NOTE: get the same results with closest, contains, current, intersects methods.

CLARIFICATION UPDATE: I have working code using a different method. The question here is not how to accomplish the objective, but rather why didn't this particular method work as expected?

EXPLICITLY:
(a) WHY DID THE TESTS FAIL?
(b) HOW TO DO THIS PROPERLY WITH DateTime::Set?