Good Day,

 I created a spreadsheet to where when I enter a date in D2 it will automatically enter a due date in E2, which is 180 days out. I used =D2+180 and works. As it reaches the due date it will turn red. I used format only cells with cell value less than =NOW(). My question is how can I make E2 turn to yellow 45 days before the due date. Thanks in advance.

Add a rule that colors the cell yellow if its value is less than =TODAY()+45

Then select Conditional Formatting > Manage Rules... and move the new rule down below the original one.