They should have the same result, but different functions are involved. Without the parens, the `+=` on the right adds the two `Duration`s, whereas with the parens, it's being added to the `SysTime` which is returned by `ref` from `SysTime`'s `+=`. And because the test is supposed to be testing that the `ref` return on `SysTime`'s `opAssign` works correctly, I added the parens to make sure that both `+=`s call `SysTime`'s `opAssign` instead of `Duration`'s `opAssign`.