LocalInstant.SafeMinus gives incorrect result around the start/end of time #957
Description
The call to PlusSmallNanoseconds should be MinusSmallNanoseconds.
Discovered while adding test coverage. The fix is trivial - we should probably backport to the 2.2 branch, but it's sufficiently corner-casey to mean it's probably not worth doing a 2.2.1 build just for that.