"[E]ver since the Big Bang, time has only moved in one direction. We experience time as flowing from the past to the present and into the future, never doubling back or changing course. This is completely unlike our experience of three-dimensional space, which we can move in freely."
. . . .
Intuitively, we explain what is happening now by referring to what happened before: events on Tuesday explain events on Wednesday. But this creates a problem when thinking about the Big Bang and the early Universe. "We can only explain things by looking at earlier things, so of course you can't explain the initial state," says Adlam.
The most important thing, Adlam argues, is for the history of the Universe to be consistent, without any contradictions or paradoxes like people going back in time and killing their grandparents (see The dangers and paradoxes of time travel). That is another reason why we should only experience a one-way flow of time – it minimises opportunity for such paradoxes. But the underpinnings of this internally-consistent history might be counter-intuitive.
People often think of the Universe as moving step by step from the past to the future. "You give it an initial state," says Adlam. Then, like a computer, "it does a computation and produces the history one step at a time, in some kind of ordered process."
But maybe that's not how it works. "What you really want to think about is the Universe deciding the whole of history all at once," says Adlam. In other words, it's not just the past that's fixed: the future is too, we just don't know what it is yet.
Michael Marshall, How did time begin, and how will it end?, BBC, https://www.bbc.com/future/article/20231115-how-did-time-begin-and-how-will-it-end (16th November 2023)