Maybe there's an algorithm that determines the start date. For example, March 15 is a Sunday, maybe it picks the preceding Sunday. Or if you take the numeric representation of March 15, 43095, modulo 14, you get 1. If this is the rule changing the bin size to 15 should move the start to the 16th. The binning dialog shown in the Microsoft documentation looks different from the one you show; do you have the most recent version installed?
https://docs.microsoft.com/en-us/pow...ng-and-binning
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