Calculated field based on other fields input

Hi Experts,

I'm creating a SharePoint form. One of the fields on the form (Priority field) should not be an input field but rather a calculated field based on two other DATE fields. The value of the Priority field can be one of four option:

If (DATE1 - DATE2) <= 7 days
 Priority = "1"
Elseif  (DATE1 - DATE2) <= 30 days
Priority = "2" 
Elseif  (DATE1 - DATE2) <= 60 days
Priority = "3" 
Else
Priority = "4" 

Endif

Could you please help me understand how can I use JS to implement it in my form? or is there an easier way to do it?

Thank you.