SPX 500 Monthly Return Google Finance

I need help with my stock spreadsheet, basically I need to formulate the SPX500 return from x start date to x end date, I have managed to figure out x start date to x current date (year to date return) but I cant work out how to do historical to fixed date, hopefully someone can help me.

Here’s what I have thus far for year to date yield.

I need to work out returns from 1/1/21 to 31/1/21 for the S&P500