A sequence of two patches is attached.  The first fixes the ~2/~4 bug,
signalling an error for any unrepresentable offset.

The second is a bonus patch, which fixes related problems in ~z, the
RFC 822 zone format specifier.  Prior to the patch, ~z outputs "Z" for
UT, which would be correct for ISO 8601 format but is deprecated (along
with all the other single-letter syntax) for RFC 822.  The patch changes
that to the approved "+0000".  ~z also had exactly the same problems as
~2/~4 regarding unrepresentable offsets, so the patch fixes them in the
same way.

I could report the ~z problems in a separate ticket if you like.
Beware that the second of these patches has some textual dependence on
the first, so trying to handle them separately might just be confusing.

-zefram