Did all 3 dimensions and time spring into existence at the same instant?
Or was it point, then line, then plain, then finally "thickness/time"?
Everything was flat (a plain) until "thickness/time" kicked in. :^)
No matter what theories are extant at this”time”, time must have always existed. To say that “time began at the Big Bang” delineates a period before and after the Big Bang. Thus, however measured, “time” must have existed pre- and post- Big Bang.
N’est pas?
When Physicists say flat, they mean that space is Euclidean.