As reported at Grinnz/Time-Moment-Role-TimeZone#1. DateTime::TimeZone::OlsonDB::Rule loads DateTime::Duration but doesn't use it; DateTime::TimeZone::OlsonDB::Observance loads and uses it, and DateTime::TimeZone and DateTime::TimeZone::OlsonDB use it without loading it. DateTime::Duration is part of the DateTime distribution (thus a dependency on it would be circular and undesirable).