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    The law requires virtually all male U.S. citizens (regardless of where they live), and male immigrants residing in the U.S. (permanent resident aliens), to register within 30 days of their 18th birthday.   Therefore, to be in full compliance with the law, a man turning 18 is required to register during the period of time beginning 30 days before, until 30 days after his 18th birthday...a 60-day window.