Without even looking at the patch, IMO it would be much better to add tm_gmtoff and tm_zone (and any other fields) to the record returned by localtime(), but in such a way that when accessed as a tuple it still has only 9 fields.  There is infrastructure for doing so somewhere for the stat structure that I'm sure could be borrowed or generalized (if it isn't already general).  That's much better than adding a new function.