Issue with current documentation:

I have three syntaxes that attempt to convert a timedelta64 in month into days. As is well explained in the documentation:
https://numpy.org/doc/stable/reference/arrays.datetime.html#arrays-datetime
this is not possible because we do not know how many days a month holds:

np.timedelta64(1, 'M') / np.timedelta64(1, 'D')
Cannot get a common metadata divisor for Numpy datetime metadata [M] and [D] because they have incompatible nonlinear base time units.

timedelta64(timedelta64(1, 'M'), 'D')
Cannot cast NumPy timedelta64 scalar from metadata [M] to [D] according to the rule 'same_kind'

but this yields:

timedelta64(1, 'M').astype(timedelta64(1, 'D'))
numpy.timedelta64(30,'D')

I am surprised that there is no error in the latter case but I am mostly interested to find out which rules are used to determine 30 in this case. For example:

timedelta64(3, 'M').astype(timedelta64(1, 'D'))
numpy.timedelta64(91,'D')

which indicates to me that there is some kind of heuristics used but I have not been able to find its documentation.

Thank you for your help,

Sacha

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