Q:

Conditional formatting on google sheet

I need to format A1 when

B1 is empty and A1 is lower than today

A1 is a date, like a deadline that I will print red if the deadline is not reached. B1 is the cell used to say it was done.

A:

Go to conditional formatting and use the custom formula =and(A1<today(),B1="")
You can set the background color red there too.
EDIT: to extend the formatting down the column but not beyond where you have date, you can do apply to range A:A and amend the formula to =and(isDate(A1),A1<today(),B1="")
Here is a picture 

