Fogg had thought it was one day later than it actually was, because he had forgotten this simple fact. During his journey, he had added a full day to his clock, at the rhythm of an hour per fifteen degrees, or four minutes per degree, as Verne writes. At the time, the concept of a "de jure" International Date Line did not exist. If it did, he would have been made aware that it would be a day less than it used to be once he reached this line. Thus, the day he would add to his clock throughout his journey would be thoroughly removed upon crossing this imaginary line. But a "de facto" date line did exist since the UK, India and the US had the same calendar with different local times, and he should have noticed when he arrived in the US that the local date was not the same as in his diary (his servant Jean Passepartout kept his clock set to London time, despite the clues from his surroundings).