Tyler is correct that you either need to use in pipeline or an automation to do this. The formula you posted will always return the current date once the check box is checked. It will not be frozen.
As you posted, if all of your editing was in a form then you could use a form rule but once you get into great at it then you need to use an automated process which kicks in after the record is saved.
If you're not up to learning pipelines immediately you can do this with an automation knowing that end of life for automations has been announced in sometime within the next 16 months or so you will have to migrate this to a Pipeline.
I suggest setting up an automation to do this and worrying about the pipeline conversion later.
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Mark Shnier (YQC)
mark.shnier@gmail.com
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