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Guild Wars 2 rewards you with a Mystic Coin if you complete a set of Daily Achievements. As their name might indicate, they reset every day, allowing you to complete them again for more arena net points and loot!
At what time does this occur?
During all of the beta weekends and as I've experienced during pre-launch weekend, the day rolls over at Midnight UTC.
Note: Some people have noticed that their achievements appear not to be resetting at rollover. Logging out to the character screen and logging back in will fix this.
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