Actually it won't - the datetime will be the same in both cases, which is why I had to compare an apparently pretty meaningless number at all. That said, it would suggest that the test is not as it could be, which is fine 'cos I'm just about to push a change to `_common()` that means it can take a coord name as an argument so we can check the epochs of `forecast_reference_time`, for example, are also being changed correctly by the function.