Show date field that's 90 days from $date_entered

Our proposal contracts expire in 90 days, so I want a field that shows the expiration date. It seems simple but the formula... 

addDays($date_entered,90)

...isn't working. The field comes back as blank. Using $date_entered as the date, how do I add 90 days, then have that date displayed? What kind of field should be using(e.g. date or datetime)?