In cell C1 use the formula =INT(B1-A1) to get the days. Then you have to get the remaining hours, which is the formula =MOD(B1-A1,INT(B1-A1)) and represents the remaining time. So in cell D2 enter the formula =HOUR(MOD(B1-A1,INT(B1-A1))). In cell E2 enter the formula =MINUTE(MOD(B1-A1,INT(B1-A1))).
You can opt out of Medicare Part B and in some situations you should opt out. Most people are automatically enrolled in Medicare Part A and Part B when they turn age 65. And most receive Part B at no charge. Medicare Part B is optional and you may not need the coverage if you have a group health plan.
Once you become eligible for Medicare, you can no longer get a subsidy. If you keep the plan anyway you will get that late Part B enrollment penalty. Enroll in all parts of Medicare and cancel your Marketplace plan.