Enrollment is not retroactive, and it cannot be made effective the day you enter on duty as you must have been in a pay status during some part of the pay period which precedes the one in which your enrollment becomes effective. Once this requirement has been met your enrollment will become effective on the first day of the first pay period that begins after your employing office receives your enrollment request. Thus the earliest your health insurance can possibly become effective is the beginning of the pay period that begins after the pay period in which you are hired. You cannot be reimbursed for any medical expenses incurred prior to the effective date. You need to consider this in cancelling any other health insurance coverage you may already have, and for scheduling doctor visits or tests.