As long as it's documented well, I don't think it should matter. If it doesn't do the thing you expect, don't use it. I'm assuming there's almost a 1:1 association between devs who will use Duration::from_days() wrong and devs who would make their own constants and use them wrong, so the least harmful approach is to make them available and document them clearly.

But yeah, this topic has been beaten to death and I'm mostly whining about a bit of typing, which is now substantially less with Duration::from_hours() being stabilized. I'm happy they stabilized minutes and hours, and I'm hoping they stabilize days at some point.