UTCDateTimeAttribute accepts timezone-naive datetime objects.

[abcydybcy]

Not much of a bug, but stumbled upon it today and wasted a good while. Might be reasonable to consider throwing a warning when UTCDateTimeAttribute is assigned a timezone-naive value.

[ikonst]

It might be a good idea -- wasted plenty of time myself over this (naive when setting for the first time, UTC when reading -- and then arithmetics blow up).