@J0hn
So it is an Excel file you want to update? You originally mentioned that you had a SharePoint list that you wanted to update. Can you clarify which you are doing?
Your lists both show months on them. I assume that is the relationship between the two is significant.
You have the month shown on the top of your screen image. That would be relevant. How is that populate? What is the Text property of that label?
Essentially what you are looking for will be this, on the OnCheck action of your checkbox:
UpdateIf(yourList, Month=yourlabelformula , {NameofTheColumnToWrite: Now()})
I would also recommend this in your OnUncheck:
UpdateIf(yourList, Month=yourlabelformula , {NameofTheColumnToWrite: Blank()})
This will reset the Excel value if the person unchecks the toggle.