Date Formula

Well, it's not a formula. But it's easy to do. If you are 100% sure that the checkbox is being checked on a form then you can use a form rule. When I say a form I mean, as opposed to say grid or some kind of data import process.  

in Legacy Forums, (Old forms), there is a rule that when the record is saved, and a certain field has  been changed, then set a date to the current date.

I don't think they have that same support for when the field has changed. But you could still have a form rule that says when the record is saved and the checkboxes tracked and the date field is blank, then change the date to the current date.  

Now, if it's possible that the checkbox is being checked in form rules, of course do not apply because form rules are for use on forms.  In that case, you would need to use a pipeline that would trigger when the checkbox is changed to checked.

The low tech way to use the pipeline is to create a native Quickbase field called [Today] with. Formula of Today().  Then use that in the pipeline to populate your date field.