Why should I include second payment of1200? I didn't receive second payment until Jan 4, 2021?

[SweetieJean]

Because the law requires that you do.  Both stimulus #1 and #2 are advance payments of a 2020 tax credit.

[xmasbaby0]

Congress passed the 2nd stimulus bill at the end of 2020. Both stimulus checks were an advance credit for your 2020 tax return,  The 2nd stimulus checks started going out at the very end of December 2020 and continued into early January 2021.  When you fill out the recovery rebate credit it is important for you to enter the entire total of the amount you have received for yourself, your spouse, and your dependent children so far.   Entering the wrong amount will delay your 2020 refund.

It should help you if you bear in mind that the purpose of the question is to determine if you have already received the amounts you should have received.