The formula i am using for a certain date/time field keeps changing when any change is made in the module. What do i need to add to my formula that puts in the day of a dependent checkbox being marked.

We have a checkbox in a module that when checked - sets the date to "today". However, when anything else is changed in that module, the date of the field updates to the new date. We want it to only set once, when the related checkbox is marked. What do i need to add to our logic that fixes this issue?