How can I take a date+time field and add 2 days to it in a calculated field to get an alert 2 days after that date+time?

I would like to use a calculated field in Alerts to notify me 48 hours after a date/time (listed in a date/time field) has passed. This would save me the trouble of having to manually impute the "2 days after" date into a form every single time. Is there a way to do this?