In the seven clean days, we count days and not hours. In the case of international travel during the clean days, the day of travel halachically stretches to ‘gain’ or shrinks to ‘lose’ the hours along with the traveler.

For example, if you make your hefsek taharah in Europe on Monday, and travel to Los Angeles in the interim, you may only immerse in Los Angeles on Monday night local time. This is the case even though that is several hours later than when you would have immersed had you remained in Europe.

(Were a woman to travel in the other direction, the reverse would apply.& In other words, a Monday hefsek taharah in Los Angeles would be followed by a Monday night immersion in Europe, even though it would be several hours earlier than her immersion had she remained in Los Angeles.)