Words denoting time are compounded when in construction, with words denoting the object whose duration is measured by the time, and the compound is tat-puruṣa,
This is also a kind of genitive compound. As मासो जातस्य = मासजातः 'a month old' (born a month ago.) So also संवत्सरजातः 'a year old'.
द्व्यहजातः 'two-days' old.' &c.,