Economic impact statement dated 2021

I don't think it matters to my question, but I am an American citizen with a tax residence in Japan.


The only payment I received was from the former president, and the check was dated January 6, 2021. 
I assumed that since the check was dated and received in 2021 it had no effect on my 2020 return, but it occurred to me (after I filed, of course) that I might be wrong.

Should I have declared that I received a $600 stimulus when filling out my 2020 return?
Or was I correct in assuming that a check dated 2021 has no effect on my 2020 return?
And if I should have declared it, should I file an amended return or just settle it later if I get flagged for an error?

Thanks for your time. 

Edward Lipsett