jabowery has asked for the wisdom of the Perl Monks concerning the following question:

I must not understand how subtraction works with DateTime. Using one method, I get 4 days and using another, apparently equivalent, method, I get 5 days duration: 
main::(a.pl:49):    my $first_time_date_time=DateTime::Format::ISO8601
+->parse_datetime($first_time);
                                                                      
+                                                                     
+                    DB<2> p $first_time
2013-08-01T20:10:31
                                                                      
+                                                                     
+                    DB<3> n
main::(a.pl:50):    my $last_time_date_time=DateTime::Format::ISO8601-
+>parse_datetime($last_time);
                                                                      
+                                                                     
+                    DB<3> p $last_time
2013-08-06T20:09:34
                                                                      
+                                                                     
+                    DB<4> n
main::(a.pl:51):    my $duration=$last_time_date_time->subtract_dateti
+me($first_time_date_time);
                                                                      
+                                                                     
+                    DB<4> 
main::(a.pl:52):    my $days=$duration->days;
                                                                      
+                                                                     
+                    DB<4> p $duration
DateTime::Duration=HASH(0x1a2f268)
                                                                      
+                                                                     
+                    DB<5> p $duration->in_units('days')
4
                                                                      
+                                                                     
+                    DB<6> x $duration->deltas()
0  'months'
1  0
2  'days'
3  4
4  'minutes'
5  1439
6  'seconds'
7  3
8  'nanoseconds'
9  0
                                                                      
+                                                                     
+                    DB<7> $d=$last_time_date_time->delta_days($first_
+time_date_time)

                                                                      
+                                                                     
+                    DB<8> p $d
DateTime::Duration=HASH(0x1e1e2e8)
                                                                      
+                                                                     
+                    DB<9> p $d->in_units('days')
5

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Re: Inconsistent results of subtraction with DateTime?
by rjt (Curate) on Aug 07, 2013 at 19:37 UTC

    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", $_, $sub->{$_}, $days->{$_};
    }

    [download]

    Output:

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

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    use strict; use warnings; omitted for brevity.
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Re: Inconsistent results of subtraction with DateTime?
by Anonymous Monk on Aug 07, 2013 at 19:08 UTC

    Aug 6 is 5 days later than Aug 1 (Delta_days)

    Aug 6 @ 8:10 is 4 days and 23.98 hours later than Aug 1 @ 8:09 (time subtraction)

    ... Makes sense to me. You should check the documentation to be sure.

[reply]
      Ah, thanks. I had made the mistake of thinking that the times were identical between the two datetimes (I didn't even look at the output of the 'p' debugger commands I was so "sure") when, in fact, they were offset from each other by some minutes.
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