Those of us, non-physicists, we do seem to realize that time is eternal. Yet there was a time when time did not exist; tough to comprehend? For us, time is something running on a clock or tracked on a calendar. Let me share what I think about when time did not exist.
A clock or calendar is a relative measure of time. By relative I mean we choose a zero or a reference point for our time axis. For instance, the year zero, anything after is AD and before is BC. A species on a different planet or in an alternative universe may very well have taken a different zero, or altogether have invented a different way to track time. Nevertheless, our scientists were clever enough in laying out the time axis aligning with the fact that time is eternal. One can only imagine as far back in time before the year zero (negative infinity sort of), and hope there is a lot more time waiting for us.
However, how do we realize the fact that time did not once exist? Only if we choose not to restrict a dimension in a clock! Let us rethink time outside of our clocks. Time is an instant when an event occurs. We measure that it in seconds, minutes and hours or days, months and years. Again, it is our (human) measure of time. In other words, time is a dimension that captures the occurrence of events – something that happens.
When did things start to happen? When did events start to occur? Per mankind’s understanding, it is only after the Big Bang. Before the Big Bang, nothing happened, no events occurred. There wasn’t nothing to capture, so to speak. Hence they say time did not exist before the Big Bang.
P.S: I am not a physicist. So my explanation may not be scientific precisely. However I think it would suffice for non-physicists to begin understand and wonder about time.