Are you kidding?
The time difference is something made up by humans because of the rotation of the earth. the day starts at a different time depending on your place on earth. First of all, you don't have to jump on a plain, jsut cross the Spain/Portugal border. Second of all, this isn't really timetravel is it... You're still in the same time... you just call it differently because of science.
If you go to a country where they use the muslim year calendar, you travel through time too? According to your "theory", yes. And that just doesn't make any sense in my opinion.
peace!
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