[AndyJackson]

But first, yes, 31 days not 30. Q erred

Not necessarily. "30 days" is often taken to mean the time between two numerically identical dates in successive months irrespective of whether the number is actually 30 days, or 31 days or 28/29 days.

[Hostage]

Yes, thank you. Precision of 31 days is superfluous except it can confuse followers expecting precision.