Before children acquire the precise definitions of time words, like minute and hour, how do they interpret them? And how are such proto-meanings acquired in development? Here we present three experiments, and assess children’s early understanding of seven time words: second, minute, hour, day, week, month, and year. Our findings indicate that children first learn time words as a lexical class, then learn their ordinal relations, but initially have little to no knowledge of their relative durations. This understanding emerges late in development – many years after children first start using time words in speech – and in many children does not emerge until they have acquired formal definitions for the words.