This appears to be new, at least I noticed it today:
I generally use my own formulas, but I think networkingdays has always been working like it does now.
The formula guide shows a sample with a calculation just like your sample, but the explanation (“between”) contradicts that. I guess once you know, it is easy to use the formula in a way that works for your purpose. Personally, I like the way it works and I would prefer for the formula guide to adjust the explaining text.
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It’s inclusive in either direction 
So giving it the same day is 1 day (that day), giving it the previous day is -2 days (this one and the past one, moving backwards hence the minus)
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Alright thanks all, somehow I missed this as the previous behavior. Odd behavior but obviously it would break documents to change it, so thanks for pointing this out and I can just adjust my document.