The related change didn't sufficiently prepare the tests for
timestamps that have offset hex parts. When adding a delta to a
timestamp we need to preserve the original timestamp's hex part and
adjust the float part by the delta amount.
Related-Change: Id214c965c4fd6f96889fc2fd4ec709cb8987726a
Change-Id: If8e98c5e088cecf44805bb6c3d1f20a2b6fc56c2
Signed-off-by: Alistair Coles <alistairncoles@gmail.com>