How to add the function to change the colour of a cell on numbers after a certain date has been passed
This is a good use for Conditional Highlighting.


Select the cells that contain your date values.
In the Inspector select the Cell tab
With all the cells still selected, click the Conditional Highlighting... button.
You will select Add a Rule...
Add rule Date is before... Date is before the date 1 days ago | Red Fill.
Add rule Date is after... Date is after the date 0 days from now | Green Fill.


Experiment with custom styles to get the look you want.  This could also be expanded with additional rules to fine tune.  For example changing the colour to Yellow if the warranty expires in the next 2 months.

For further information on how to use Conditional Formatting check out Apple's Support Documentation.