DateTime says:

• date vs datetime math

If you only care about the date (calendar) portion of a datetime, you should use either delta_md() or delta_days(), not subtract_datetime().

In short, subtract_datetime gives you an answer that includes days, minutes, and seconds between the two times. But delta_days essentially chops off the "time" part of your DateTime object and then does the subtraction, so you only get days.

To see for yourself, try dumping the DateTime::Duration objects:

    use DateTime;
    use DateTime::Format::ISO8601;

    my ($first, $last) = map { DateTime::Format::ISO8601->parse_dateti
+me($_) }
                          qw < 2013-08-01T20:10:31 2013-08-06T20:09:34
+ >;

    my ($sub, $days) = map { $last->$_($first) }
                         qw< subtract_datetime delta_days >;

    printf "%12s   %5s | %-5s\n", 'Value', 'sub', 'days';
    printf "%.21s+%.8s\n", ('-'x21)x2;
    for (qw< months days minutes seconds nanoseconds >) {
        printf "%12s : %5d | %-5d\n", $_, $sub->{$_}, $days->{$_};
    }
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Output:

       Value     sub | days
---------------------+--------
      months :     0 | 0
        days :     4 | 5
     minutes :  1439 | 0
     seconds :     3 | 0
 nanoseconds :     0 | 0
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