I am new to Google Sheets, excuse me for a (most likely) dumb question.
I have a column with dates looking like
What formula do I use to fill the cells with values not equal to today date in red?
You need to go to Format\Conditional formatting. In new window add new formatting. Then select range to which you need to apply formatting and set Format rules "Date is before" "Today" and select Formatting you need - red fill. Then do the same procedure but for Format rules "Date is after" "Today".