Relative tense is a tense that refers to a time in relation to a contextually determined temporal reference point, regardless of the latter’s temporal relation to the moment of utterance. (http://languagelink.let.uu.nl/tds/onto/LinguisticOntology.owl#relativeTense with reference to http://www.sil.org/linguistics/glossaryoflinguisticterms/whatisrelativetense.htm)