Submitting Disability Continuation Form Late

I’m on disability due to pregnancy. My doctor extended my disability, but it took them a long time to fill out the paper form for me to mail back. The form says to return it 20 days from the date they mailed it to me. I’m mailing it out today, 19 days after the mailing date on the form. Since it won’t arrive back at EDD within 20 days, will I have a problem with the claim? Is it 20 business days or just 20 days? Or does it just need to be postmarked within 20 days? Thank you!