Hi john white!
You are correct - you can look to use a parameter where the user can first select from here: Most Recent or Custom, and after click Custom they can then use a Date Picker.
I first find the Max Date in the data using a LOD expression. If they select Most Recent, I use this field, otherwise I use the Date Picker (which I compared to Max Date to ensure it's not more recent than the max date - if so, I "snap" it back to the Max Date.
I've attached an old date template workbook for reference. I'm sure I've learned new/better ways since doing this, but figured it might help!
DateFunctionsTemplate.twbx 56.2 KB