The only definite dating we have for the ancient day names of the Hobbits is in Appendix D to The Lord of the Rings, where we're told that they were in use 'at least nine hundred years before Frodo's time', hence the approximate date of III 2100 above. The fact that the words have a commonality with the language of the Rohirrim implies that their roots went back much farther, to the time when the ancestral Hobbits still dwelt alongside Men in the Vales of Anduin (though the precise form of the name Sterrendei probably underwent some evolution over that period).

Strictly speaking, we only have direct evidence of the use of this old form in the Shire itself; for Buckland and Bree, we can only infer its likely usage. Buckland was founded in the Shire-year 740, a point in time about two centuries after this old form of the name was recorded and so, at least in principle, it might already have mutated to Sterday before the first Bucklanders settled their land. In the case of Bree, we know that the newer 'modern' forms of the day names were used there, and given that Bree was older than the Shire, it seems a reasonable presumption that the older forms were also used there historically, but this not explicitly stated.