Time laid out so that days can be pointed at. A calendar makes duration addressable.
Latin calendarium (an account book), from calendae/kalendae (the first day of the month, when debts and interest were due), related to calare (to call out, to proclaim) — the same root behind English 'class.'
Time laid out so that a particular day can be pointed at and found again — duration made addressable rather than merely felt passing.