Opportunity Validation Rules

Here is a validation rule on the opportunity that is critical to making sure all opportunity close dates are current and up to date!  

I'll start with the error message so you can see the intent:
"Open Opportunities cannot have a Close Date in the past. Closed Opps cannot have a Close Date in the future. Please update your Close Date. Thanks!"

The goal here is to make sure that Close Dates are always current for open opps (meaning they can't be in the past!) and Closed Opps cannot have a Close Date in the future (you've seen it before, an Opp Closed Lost with a Close Date 2 months from now... that's just bad data integrity!  

Here's the rule to keep it clean!
Formula:
OR(

AND(
CloseDate < TODAY()- 3,
IsClosed <> TRUE
)
,
AND(
OR(
TEXT(StageName) = "Closed Lost",
TEXT(StageName) = "Closed WON"
),

CloseDate > Today()
)
)
End of Formula

Now, you'll notice that the rule is "CloseDate < TODAY()-3".  I did that to give some flexibility to the sales team as they might be working a deal and need to make changes or waiting for something on a Sunday and need an approval so I wanted to be a bit forgiving.

But otherwise this will work well to keep those Close Dates up to date as a Rep cannot save their Opp unless the Close Date is adjusted to meet the rules.