Conditional format where cell value is a past date

you can use conditional formatting to fill the date when it is 3months to todays() date

use a formula
=cell with date in < DATE(YEAR(TODAY()),MONTH(TODAY())-3,DAY(TODAY()))

assuming your data is in C2:E200
select that range and then use
= C2 < DATE(YEAR(TODAY()),MONTH(TODAY())-3,DAY(TODAY()))


for 2007, 2010 or 2013 excel version
Conditional Formatting

Highlight applicable range >>

C2:E200


Home Tab >> Styles >> Conditional Formatting
New Rule >> Use a formula to determine which cells to format
Edit the Rule Description: Format values where this formula is true:


= C2 < DATE(YEAR(TODAY()),MONTH(TODAY())-3,DAY(TODAY()))


Format… [Number, Font, Border, Fill]
choose the format you would like to apply when the condition is true
OK >> OK


Otherwise
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I usually use the DATE() function - which does look complicated
but all its doing is finding the date() from the cell
using year(), month() , day()
and then i subtract the number from one of those items
OR you could just take of 90days , if you not worried about accuracy, ie feb has less days etc

= C2 < today() -90