The bible doesn’t say Jesus will be reborn of a woman in the same fashion He did first time around, nor does it say around what time it will happen.
The texts also didn't say the Messiah would born of a virgin the first time, that was retroactively added involving a prophecy about a young woman (almah is a young woman of childbearing age, no where else is that word translated as virgin even when in some cultural spaces it referred to one who hasn't had a child yet - those aren't the same thing) that came true later in the same section (that the threat to King Ahaz would end before the child was weaned and old enough to reject bad and choose good), but has since been called a 'double prophecy' to handwave that issue. Something people who studied the texts should know.
People have used and refitted the texts to prove other messiahs for centuries. While it could be useful to explore the thought experiment of messiahs and demigods of all faiths, if we're going to stick purely to the texts, then we have to acknowledge the texts were bent to fit Jesus (or later writer's ideas about him and many other people) first.
Jesus Christ is the single most significant person to have ever been born. If He was not, we would not tell time by it.
Time hasn't been split in two, it still continuously flows. Some societies have created social constructs, but that came from the Roman system and it spread due to European empires, but it isn't universal by a long shot. It's currently 5781 in the Hebrew calendar (some Messianic Age types - Jewish, Christian, and those influenced by them - use the '"A thousand years in your sight are like a day that has just gone by," verse to think something will happen around 6000 as it that would signify going into the Sabbath, often leaving out the end of that verse is "or like a watch in the night"), 12020 in the Holocene calendar, among the dozen or so other options available. Culture alter our perception of time, but it can't really split or change the fabric of time. We don't have enough mass for that.
Those who don't believe in Jesus. I take it you don't celebrate Christmas then.
No, I don't. I haven't since I left home nearly twenty years ago. Don't have a single good Christmas memory so it wasn't worth keeping in my life. I did spend a good ten years after that studying religious texts along the sociobiological impact holidays and traditions have and while connection to tradition and through that community and self of self can be an important part of a holiday, personal belief rarely is a defining factor over a culture as a whole.
People can make holidays mean dozens of things, people very much can have it all the ways. There are many traditions of ancestor worship or grief rituals like sitting shiva that, while some definitely believe, many people carry them out even without those beliefs for the connection they give or other benefits. Christmas can have benefits without religious belief involved.