Have you ever come across Zeller’s formula (aka Zeller’s congruence)? If not directly, you will have indirectly.
For every digital device you have that shows the day of the week, whether your phone or your computer, there’s a chance this is what powers it. Or a variation of it, at least. Created by Christian Zeller in the 19th century, it’s a set of formulas that let work out the day of the week, based on the provided date.
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