LocalTimezone support would be *really* helpful for the email module.  It would allow us to have unambiguous semantics for datetime objects representing timestamps exacted from or inserted into email messages (see issue 665194 for recent discussion).  The email module is already trying to handle timestamp translation, and I'd be willing to bet it is buggier than the proposal here.

At one point I even started to copy the LocalTimezone class from the docs into the email module.  I implemented a naive extension of the current formatdate function instead, but after Alexander's feedback on #665194 I think the naive implementation is not a good idea.