Same as I mentioned on the who. Usually it will depend on the people around them, the people they knew outside of this world, but even the people they meet here sometimes. But for all those cases, you have an exception like... well Portgas again comes to mind as he's died, and been brought back at many different points. Occasionally with them going several years back in his history.
The only theory I have on that is due to my world's timeline. Portgas is at the turning point of several major world events in my world, including the end and start of two different eras. So perhaps they use him to toy with everyone else's time period. Whether there are people like that for their worlds, I could not say. I don't know of anyone else off the top my head who has come and gone that many times or from that many different time points, though it might be worth looking into.
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The only theory I have on that is due to my world's timeline. Portgas is at the turning point of several major world events in my world, including the end and start of two different eras. So perhaps they use him to toy with everyone else's time period. Whether there are people like that for their worlds, I could not say. I don't know of anyone else off the top my head who has come and gone that many times or from that many different time points, though it might be worth looking into.