What insurance company? I have Unum Provident, and I'll explain how they operate. I was approved for SSDI on first application, so no retro pay. Of course, there was the five month waiting period. December was my first onth for SSDI, however, I did not receive it until January. Unum did not pay me anything in December. Stating they always paid for the previous month like SS. Of course, total BS.
However, they did subtract the amount they were paying from my SS to equal what I had previously received from UNUM. i.e. UNUM paid 1200 a month pre-SS. I was awarded 800 from SS, so Unum pays me only 400. Now, had I been entitled to retro pay from SS, UNUM would expect whatever they paid while I was eligible for SS. i.e. I was entitled to 12 months retro, Unum would expect the 400x12=4800. After that they would continue withe $400 payments.

