See mod/data/field/date/field.class.php :

function get_sort_sql($fieldname) {
return 'CAST('.$fieldname.' AS unsigned)';
}

This causes problem at PostgreSQL as there is no such type "unsigned": ERROR: type "unsigned" does not exist. Note that CAST() is SQL standard but the target datatype has to be known for the SQL environement.

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Assigning this to myself as a part of my Bugathon run.