Trying to determine if there is any benefit or pitfall in timing medicare enrollment. Appreciate any experience shared here. So, I know that Initial Enrollment Period starts 3 months before the 65th birthday and ends 3 months after the 65th birthday. Example: Person born last day of Sep. Can enroll Jul,Aug,Sep,Oct,Nov,Dec Assuming current coverage under ACA and I already know that I can have both ACA and Medicare during the transition and I assume that if enrolling in December, with just a few days until January starts, Medicare may take a little time to kick in. In my case, that transition would amount to a 24 hour period (having both ACA and having enrolled in Medicare A,B). In reality, I would likely not wait until last day of December and would enroll some day during December. I read healthcare.gov/medicare/changing-from-marketplace-to-medicare/ What I don't know and am unsure about is : Assume some type of need for medical care, emergency, how does one go about presenting proof of insurance on January 1 of the new year if Medicare enrolled in December the week before ACA terminated ?