After 5 days after the time has been changed everything will return to normal.
Alternatively, a safer way of changing the time is from within the game itself.
It acts the same as if the system time was changed, but things will come back to normal after 24 hours to the next morning,
- The player might be unable to collect donations from the fountain.
- Daily specials in stores may have the same item daily.
- Seasonal items may disappear.
- Time traveling can be considered cheating, so the cooldown time for items and such to work is probably so the feature of changing the time isn't abused by the player.
- The term time traveling is also very common in the Animal Crossing community.