extension / gap question

Thank you in advance. My disability period ends in a few days. I expect to receive the extension medical form in the mail from them in a few weeks; this is not my question. The doctor who did the original/current certification wants a different doctor, a specialist to do the extension paperwork but I will not be able to see that doctor until next week. If this second doctor does not back date the form to the last day of my approved disability, will I loose the ability to get benefits under an extension? Reading in Reddit that there might be gap days, that would be fine. How does this all work? again, thank you.