Ignorance is bliss. Most projects and even many formal standards don't even think to consider such questions, let alone try to answer them. X.509 builds on ASN

Ignorance is bliss. Most projects and even many formal standards don't even think to consider such questions, let alone try to answer them. X.509 builds on ASN.1, which incorporates ISO 8601, which in turn incorporates IEC 60050-111. Unfortunately, as elucidated in this thread, the relevant portion of X.509 is still underspecified--it uses the term "inclusive" without resolving ambiguity related to intervals and granularity, which it could have done by using formulas provided by ASN.1, ISO 8601, or IEC 60060-111.