https://www.polygon.com/pokemon-go/2018/4/4/17197256/pokemon-go-field-research-rewards-daily-quests-challenges-moltres

Pokémon Go’s recently unveiled daily quests give new purpose to the mobile game, promising surprise rewards each day. Those culminate in a much bigger prize after seven days of successfully completing the game’s research challenges, as players have already discovered.

Several Redditors are reporting their prizes for collecting all seven stamps, the achievement awarded for completing the daily Field Research tasks. Although the feature only launched on March 30, certain players have already managed to complete seven straight days of the daily, rotating tasks. (It sounds like players switched the time zones on their phones in order to trick the game into believing they’d played for an entire week, granting them stamps for additional days as if they had completed the research challenges then.)