Re: DateTime Subtraction

[Todd Goldenbaum]

On Jan 25, 2005, at 8:57 AM, Hill, Ronald wrote:
Hi Todd,
Todd Goldenbaum wrote:
Hello,
I'm trying to find the difference betwen two datetime
objects, but I'm
getting some unexpected results.  I start with two dates out of mysql
(code simplified  a bit for this example):
   my $start_date = DateTime::Format::MySQL->parse_date( '2005-01-05'
   ); my $end_date = DateTime::Format::MySQL->parse_date(
'2005-01-23' );
Then I use one of these two techniques to perform the subtraction:
   my $duration = $start_date->delta_days( $end_date );
   my $duration = $end_date - $start_date;
Expecting to get a $duration of 18 days.  But with either technique,
the resulting $duration->days value is 4 instead of the expected 18.
And when I ask for $duration->hours, it gives me 0.
Further experimentation with the two dates involved reveals that
subtracting two dates that are within a few days of each
other seems to
work, but at a certain point it breaks down and gives erroneous
results.  Any idea what could be going on here?
Thanks,
Todd
There are a couple of ways to do this;
use strict;
use warnings;
use DateTime;
use DateTime::Format::MySql;
my $start_date = DateTime::Format::MySQL->parse_date( '2005-01-05' );
my $end_date = DateTime::Format::MySQL->parse_date( '2005-01-23' );
my $dur = $end_date->delta_days( $start_date );
print $dur->delta_days;
gives the correct result of 18
or
using your example above
my $duration = $end_date - $start_date;
print $duration->in_units('days');
gives the same results.
I see now, that did the trick.  My mistake was assuming that 
$duration->days would give the same result as $duration->delta_days( 
$start_date ) or $duration->in_units('days').

I'm curious now though, what >is< $duration->days used for, if not for 
expressing the length in days of a $duration?

Todd