This looks like a great read. I've saved it to my library; but at 487 pages, I'll have to get back to you on this.
We know that it was accepted as early as the fourth century:
File:Beit Alpha.jpg - Wikipedia
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Hammat Tiberias - Wikipedia
However I suppose that it's possible that this was a result of syncretism.
There is also the calendar change of 701 BC, and it was important enough to be recorded three times in the scripture, (2 Kings 20:1-11, 2 Chronicles 32:24, Isaiah 38:1-8).
If the sundial of Ahaz marked the sacred calendar day then it marked the half hour as well as the hour. Ten steps would therefore be five hours. The length of the solar year at 365.24219 is 365 days and about 5.8126 hours. The shadow does not go back on the sundial without a catastrophic or semi-catastrophic event such as a small pole shift, (not magnetic pole shift but a temporary geographic shift or strong wobble).
The length of the year surely changed at this time. Dr. Chuck Missler even did a piece on this but because he viewed the flood account as signifying a year of 360 days, (the so-called "prophetic year"), he proposed that the year changed from 360 to 365.2422 in 701 BC. He however had to ignore the Egyptian calendar to come to such a conclusion: for the Egyptians knew the length of the year as 365 days, as said before, long before the time of Mosheh. There was indeed a great calendar change around that time, for something surely did happen, and it is mentioned three times in the scripture.
The year likely changed from 365 days exactly, (the same number as that of the years given to Henok in Genesis 5:23), to 365.24219 days. After our planet settled into its new course, the overall change amounted to almost six hours, (about 5.8126 hours), nearly the same amount which would be accounted for in the time of ten steps on a sundial, one which marks both the hour and the half hour for the commencement of the hours of prayer, (which commence at the half-hour).
The Long Night of Sennacherib: Mars’ Near Pass-by? – Dr. Chuck Missler – Koinonia House
Therefore, either way, whether you say the year was 364 days in the beginning, or that it is supposed to be 364 days according to the Torah: it is not the same after 701 BC even according to the scripture, even including the Prophet Yeshayah.
However, in my understanding of the Torah calendar, this 701 BC calendar change doesn't matter except that it changed the leap cycle to a more frequent period, essentially alternating between five and six years for each leap year, as apposed to the original leap cycle of every seventh year in the time of Mosheh and the giving and writing of the Torah, (if the year was exactly 365 days the leap week would be added every seventh year because on a 364 day calendar you would lose exactly one day every year).
Thus the only thing that changes on my calendar after 701 BC is the
frequency of the addition of a leap week to keep the calendar aligned with the solar year. It's pretty simple once it is understood: the vernal equinox drifts through the week days, getting closer to 1 Abib every year. When the equinox reaches 1 Abib it is a leap year so that 1 Abib always comes either on the equinox or up to seven days after the equinox. This is essentially the same rule used to keep the lunisolar calendar in sync, (1 Abib should not come before the vernal equinox because the Pesach might come too soon).