Ok,at least it prompted me to look it up n the .gov site
Changing your mind
Your COBRA coverage will be retroactive to the date that you lost your benefits (as long as you pay the premium). During the election period, you might initially decide not to take COBRA, which means you waive your right to coverage. However, as long as the election period hasn't expired, you can change your mind and revoke your waiver.
Even if you enroll in COBRA on the last day you are eligible, your coverage is retroactive to the date you lost your job, provided you pay all the retroactive premiums.
If you waive your right to COBRA but then incur medical bills during the election period, you can change your mind and elect COBRA, and your plan will cover those bills.
Conversely, if you elect COBRA, you can cancel it at any time. You don't have to use it for your full eligibility period.