This paper is the first survey of verbal affixes encoding the day period ('at night','in the morning' etc) or the yearly seasons ('in winter' etc) when the main action takes place. It introduces the term 'periodic tense' to refer to this comparative concept, explores the attested paradigms, their interactions with other verbal categories (including the more usual deictic tense) and investigates their diachronic origins. It shows that periodic tense markers are not restricted to incorporated nouns of time period, but constitute a highly grammaticalized verbal category in some languages, which redundantly co-occurs with adverbs or nouns of time.