[blindmute]

You're ignoring the method by which these things are dated. First, if a date was written on the page (extremely rare), it may well have been after the date change happened. The vast majority of writings from the time are dated to known events, like eclipses, or by reference to other known things ("20 years ago, when Joe was born" and then finding Joe's birth records).

If calendars really did advance 300 years, we are living past the advance. That means that when we see a writing reference an eclipse that we know happened in what we call 900, it doesn't actually tell us anything about the calendar having moved or not. It only tells us that something was observed 1122 years ago. That could be 900 years after 0AD, or it could be 600.

Actually, almost every instance of dating anything back then depends upon few relative date calculations which are "known". The eclipse happened 1000 years ago, so that is year 1022, and this writing referencing it is from 1022, and so this scholar writing a generation later is from 1082. This inscribed rock was found under 1100 years of dust, so the people living here were from 922, and their language existed at that time, and all records we find from this civilization are now known to be from the early 900s. Now we can claim to have evidence that there was thriving commerce in the 900s with hundreds of written records about it, so of course the dates weren't changed.

If you dig into the records people cite as evidence against the phantom time hypothesis, the vast majority of them (literally every one that I've seen) is using indirect dating in this way, which doesn't mean anything.