Updating a date field

I've run across something I thought would be more straightforward, but I can't find the resolution.

I have a Sales CRM with a field for sales stage that is populated with by selections from a list (Lead, Proposed, Closed/Lost, Closed/Won, etc.).

I have another field for "Close Date."

When a user selects the Sales Stage as "Closed/Won" or "Closed Lost," I'd like the "Closed Date" field to be automatically updated to the current date.  The only way I've attempted is using the "Today" function, but ever time I log in, it updates *all* closed opportunities to the current date.

Is there a simple solution to this?

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Michael Murphree
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