True stringify of DateTime - json

Question

According to:
http://search.cpan.org/~drolsky/DateTime-1.43/lib/DateTime.pm#Formatters_And_Stringification
the following will work:
use DateTime;
$dt = DateTime->new( ... );
print $dt; # as string
and it does.
However, when using the JSON module to embed the created $dt string as a json-encoded data structure as follows:
use JSON;
my $json = JSON::encode_json( { dt => $dt } );
The JSON module is throwing the following exception:
encountered object '2017-08-02', but neither allow_blessed nor convert_blessed settings are enabled at ...
I can work around the issue as follows:
my $json = JSON::encode_json( { dt => substr($dt,0) } );
but wonder if there's a cleaner way to get a "true" string out of DateTime?
I've looked through the DateTime docs but I don't see an explicit stringify() to create the true string as required by JSON.
EDIT: In the constructor, I specify a desired output format via 'formatter'. I'd like to get a string using the already-specified format, and not specify the format again (which would be against DRY principle...)
EDIT2: Interpolating the string is a good solution if just $dt, but doesn't work if $dt is actually a moose attribute (i.e. "$person->birth_d").

Answer 1

You can use "$dt" or "".$o->dt. Like your solution, it's short for
$dt->formatter ? $dt->formatter->format_datetime($dt) : $dt->iso8601
If you want a specific format, there's $dt->strftime, $dt->iso8601, or you could use a DateTime::Format:: module.

Answer 2

You can explicitly interpolate the object in a string ("$dt") or call the method that's actually getting called ($dt->iso8601).
use strict;
use warnings;
use 5.010;
use DateTime;
my $dt = DateTime->now;
say $dt;
say "$dt";
say $dt->iso8601;
Output:
2017-08-02T19:54:22
2017-08-02T19:54:22
2017-08-02T19:54:22