I noticed this part:

+        # We have an aware datetime.  Use aware datetime arithmetic to find the
+        # seconds since the epoch.
+        delta = dt - datetime.datetime(*time.gmtime(0)[:6],
+                                       tzinfo=datetime.timezone.utc)
+        seconds = delta.total_seconds()

Why don't you just return dt.astimezone() here?  A round trip through a floating point timestamp always makes me nervous.  I doubt loss of precision is a problem for the e-mail package, but who knows.