In ancient Greece, Plato and Aristotle agreed that the past is eternal. Aristotle claimed that time had no beginning because, for any time, we always can imagine an earlier time.
Imagine, the earliest past you have and by extension imagine all the people in history doing this. Be it as it may before mankind even.
Isaac Newton believed future time is infinite and that, although God created the material world some finite time ago, there was an infinite period of past time before that.
The thing is flow of time has no definite source or terminus. It is defined by the tethers of our existence that begin and end us.
Time and Conventionality
It is an arbitrary convention that our civilization designs clocks to count up to higher numbers rather than down to lower numbers as time goes on. It is just a matter of convenience that we agree to the convention of re-setting our clock by one hour as we cross a time-zone. It is an arbitrary convention that there are twenty-four hours in a day instead of ten, that no week fails to contain a Tuesday, that a second lasts as long as it does, and that the origin of our coordinate system for time is associated with the birth of Jesus on some calendars but the entry of Mohammed into Mecca on other calendars.
Time and Language
Using a tensed verb is a grammatical way of locating an event in time. All the world’s cultures have a conception of time, but in only half the world’s languages is the ordering of events expressed in the form of grammatical tenses. For example, the Chinese, Burmese and Malay languages do not have any tenses. The English language expresses conceptions of time with tensed verbs but also in other ways, such as with the adverbial time phrases “now” and “twenty-three days ago,” and with the adjective phrases "brand-new" and "ancient," and with the prepositions "until" and "since." Philosophers have asked what we are basically committed to when we use tense to locate an event