From high school curriculum, geography tells us that the earth isn't flat but spherical ( shape of am egg) and the difference in time zones and the rotation of the earth around its orbit proves this.
If the earth was flat, then the whole world would experience daylight and night at the same time and the the whole world would be using the same time zone.
We all know that the earth rotates, it was flat, the whole world would get the same reaction whenever the rotation occurs.