I recall that it was at some point suggested that epoch be part of dtype.  I was not able to find the reasons for a rejection, but it would make perfect sense to keep timezone offset in dtype and treat it effectively as an alternative epoch.

The way I like to think about datetime is that YYYY-MM-DD hh:mm:ss.nnn is just a fancy way to represent numbers which is more convoluted than decimal notation, but conceptually not so different.  So different units, epochs or timezones are just different ways to convert an abstract notion of a point in time to a specific series of bits inside an array.  This is what dtype is for - a description of how abstract numbers are stored in memory.