To help debug this, it'd be nice to know the platform and a recipe for
OK, here is something using only unmodified tz code:
$ uname -om
x86_64 GNU/Linux
$ rm -fr /tmp/tz/
$ make --silent TOPDIR=/tmp/tz CFLAGS='$(GCC_DEBUG_FLAGS)' -j5 install
$ TZ=Australia/Sydney ./date -r 14784296400 +%Y-%m-%dT%H:%M:%S%z
2438-06-30T23:00:00+1000
$ rm -fr /tmp/tz/
$ make --silent TOPDIR=/tmp/tz CFLAGS='$(GCC_DEBUG_FLAGS)' ZFLAGS='-b fat' -j5 install
$ TZ=Australia/Sydney ./date -r 14784296400 +%Y-%m-%dT%H:%M:%S%z
2438-07-01T00:00:00+1100