Dragon Ball isn't exactly known for its consistent storyline. The series has plenty of plot holes and discrepancies, but its main draw has always been its intense action and likable characters anyway. That being the case, a dropped plot thread here or there has never been something that was make-or-break for the franchise. Still, that doesn't mean it does everything wrong. In fact, Dragon Ball Z happened to pull off one of the most complicated concepts in science fiction: time travel.
Source：https://www.cbr.com/dbz-trunks-android-cell-sagas-sci-fi-time-travel/
