I can't subtract one DateTime object from another to get a DateTime::Duration in days

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

Oh the shame: my date objects weren't actually differing by more than one day because I'm an idiot and assumed that my inputs were what I thought they were! Thanks commenters for forcing me to go back and look at exactly what was in the objects. I leave this here as a memorial, or it can be reaped.

This is a bug in an application I'm working on.

The code creates two DateTime objects and subtracts one from the other to get a DateTime::Duration object.

However the DateTime::Duration object can't return a difference in number of days, which is what I need.

This is actually expected behaviour:

my $dur = DateTime::Duration->new( years => 1, months => 15 );
 
$dur->in_units('years');                # 2
$dur->in_units('months');               # 27
$dur->in_units( 'years', 'months' );    # (2, 3)
$dur->in_units( 'weeks', 'days' );      # (0, 0) !
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The last example demonstrates that there will not be any conversion between units which don't have a fixed conversion rate.

I guess my question is, how do I get the difference in days between two DateTime objects?

Updated: The current code just does

my $diff = $dt1 - $dt2;
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Where both DateTime objects are specific fixed dates, like ~~one is January 1, 2021 at 10 AM and the other is today at 11 AM. Surely there's a 'fixed conversion rate' in days between those?~~ Note: They were essentially the same day!

I can understand that if I create a duration object with years => 1, months => 15 that's not something where we can be sure of the number of days, but that's not what's happening in my code.

Comment on I can't subtract one DateTime object from another to get a DateTime::Duration in days Select or Download Code

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Re: I can't subtract one DateTime object from another to get a DateTime::Duration in days by 1nickt (Canon) on May 10, 2021 at 23:59 UTC
Hi, how do I get the difference in days between two DateTime objects? https://metacpan.org/pod/DateTime#$dt-%3Edelta_days($datetime) The `$dt->delta_days` method returns a duration which contains only days. `$ perl -MDateTime -E 'my $dt = DateTime->new(year=>2021); say DateTime +->now->delta_days($dt)->in_units("days")' 130` [download] Hope this helps! The way forward always starts with a minimal test.	[reply] [d/l] [select]
Re: I can't subtract one DateTime objects from each other to get a DateTime::Duration in days by LanX (Saint) on May 10, 2021 at 23:03 UTC
you are not showing any code to reproduce the problem, the snippet is just copied from the pod. My best guess is that you are requesting "days" from a duration which has "no fixed conversion rate" for days. From the docs, just a paragraph after the code you posted ... `The only conversions possible are:` `years <=> months` `weeks <=> days` `hours <=> minutes` `seconds <=> nanoseconds` Cheers Rolf _{(addicted to the Perl Programming Language :) Wikisyntax for the Monastery}	[reply]
Re^2: I can't subtract one DateTime objects from each other to get a DateTime::Duration in days by Cody Fendant (Hermit) on May 10, 2021 at 23:36 UTC
I have updated the post to make it more clear what's happening. One DateTime object, with a specific date and time, is being subtracted from another specific DateTime object.	[reply]
Re^3: I can't subtract one DateTime objects from each other to get a DateTime::Duration in days (updated) by LanX (Saint) on May 11, 2021 at 06:58 UTC
> I have updated the post to make it more clear what's happening. no you haven't, please see SSCCE for the next time. update FWIW: your approach is still wrong and will only sometimes produce correct results. See 1nickt's reply with `->delta_days` for the correct solution Cheers Rolf _{(addicted to the Perl Programming Language :) Wikisyntax for the Monastery}	[reply] [d/l]
Re: I can't subtract one DateTime object from another to get a DateTime::Duration in days by cavac (Prior) on May 18, 2021 at 10:37 UTC
From memory, DateTime is a bit limited in that regard. You can use Date::Manip for complex date math. A long, long time ago (in a galaxy unfortunately not far enough away), i wrote "Custom date calculations using Date::Manip", see 11109316 `perl -e 'use Crypt::Digest::SHA256 qw[sha256_hex]; print substr(sha256_hex("the Answer To Life, The Universe And Everything"), 6, 2), "\n";'`	[reply] [d/l]
Re: I can't subtract one DateTime object from another to get a DateTime::Duration in days by perlfan (Parson) on May 11, 2021 at 17:21 UTC
You almost always never need DateTime and can almost always use `strftime` from POSIX, in combination with the time functions perl already supports.	[reply] [d/l]
Re^2: I can't subtract one DateTime object from another to get a DateTime::Duration in days by BillKSmith (Monsignor) on May 11, 2021 at 18:11 UTC
True, you probably never 'need' the power of DateTime. However, it does offer the convenience of a 'swiss-army-knife'. It has always been able to solve my time related problems without the effort of searching for a better tool. Bill	[reply]
Re^3: I can't subtract one DateTime object from another to get a DateTime::Duration in days by perlfan (Parson) on May 11, 2021 at 19:35 UTC
I don't hate DateTime or think less of anyone who does use it, I just use it when I have to. Usually that's never. YMMV	[reply]
Re^2: I can't subtract one DateTime object from another to get a DateTime::Duration in days by Anonymous Monk on May 11, 2021 at 19:45 UTC
maybe show some code to back that up?	[reply]

update