Sforno on Leviticus 23:3/ספורנו על ויקרא כ״ג:ג׳

1'שבת היא לה, He, the creator of light and darkness assigns these days in all your dwellings, wherever on earth. The reason this is spelled out is that we might have thought that just as days and nights are not constant, being longer in the summer than in the winter respectively, so that the hours of the first Sabbath in history might serve as the model for all future Sabbaths, and Jews over the world ought to observe the 24 hours which Jews in the land of Israel observe as their Sabbath. The Torah, by writing בכל מושבותיכם, indicates that the commencement and conclusion of the Sabbath depends on the local times of day and night, not on a central location. [as opposed to the determination of the monthly time when the new moon , i.e. the new month begins, which is related to its astronomical constellation in the sky over Jerusalem, which serves as the ”dateline” for the Jewish people.Ed.]