Selected Answer
Conditional formatting is what you need and you need to use a formula for it.
Make sure you have one cell that has the certification date in it and that it is formatted as a date.
Then, select the cell that you want to change colors and go to the Home tab > click Conditional Formatting > New Rule > go to Use a formula to determine which cells to format and then enter this formula:
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=$B$1-TODAY()<=90
and click the Format button and choose yellow for the Fill color. Hit the OK button when you're done.
Follow the same steps again but this time enter this formula:
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=$B$1-TODAY()<=30
and choose red for the Fill color formatting.
In both formulas, B1 is the cell that contains the certification date, so change that as needed.