If a change compiled/built but the semantics were different would it be considered a major change?

For example, suppose a time string returned was the same format but now was CET rather than, say, IST and that interpreting it as IST now produced erroneous results would it be classified as MAJOR?

Note that I assume there is no change to the build, everything compiles and builds as before and everything runs without errors being detected just as before; its just that the 'answer' is now wrong, not right. I can't seem to find a consistent view.