I only find how to validate it as 3 distinct fields:

Then you stopped reading the documentation before you found the answer to your question

datetime => [ [ 'DATETIME_STRPTIME', '%Y-%m-%dT%T%z' ] ],
datetime => [ [qw/DATETIME_FORMAT MySQL/] ],
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Making an intuitive leap, I guess that this might also work

perl -MFormValidator::Simple -MCGI -le " my $q = CGI->new({qw/ snot 20
+11-08-11 /}); my $result = FormValidator::Simple->check( $q => [ { no
+se => [qw/ snot /] } => [ q/DATE/ ], ] ); warn $result->has_error; "
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update: after testing, it doesn't :) oh well,

perl -MFormValidator::Simple  -MCGI -le " my $q = CGI->new({qw/ snot 2
+011-08-11 /}); my $result = FormValidator::Simple->check( $q => [ sno
+t => [ [ qw/ DATETIME_STRPTIME %Y-%m-%d / ] ], ] ); warn $result->has
+_error; "
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