When claiming field research stamps, is it when they are earned or claimed that is the date?
For example, if I complete all three today but already have today’s stamp, will I get another stamp if I claim one tomorrow then one the next day etc? Or will it not pay out as I didn’t complete it on that day?
Yes. Do note that the highest Research might be a queue of Research Encounters you didn’t catch. They will not give you your Stamp, because you have already claimed them once.
No, the game knows when you completed the task.
EDIT : Im wrong
Once you complete the task, you can hold off on claiming it until the next day to get tomorrow’s stamp. I honestly practice that quite religiously so I can keep my streaks up in an event I’m unexpectedly busy the next day.
Even if the game would know, it doesnt care. You get a stamp when you claim it. Not when its completed like @Mediocre_Melvin suggests.
I use this alot when i go visit family in the weekends. Im not always able to gather research on those days so i save some from the previous day.