Calculating Date Difference In Wordpress / Php

I'm Using The Fluent Form Plugin / What i want to do is calculate the difference between the two dates .

Numeric field can be easily calculated if you have 2 numeric field , but they don't give me the option to calculate the dates .

using a custom html field below i've manged to get the value of the datetime pickers and now i'm trying to calaculate the dffernece between them but still no succes .

if you have any idea or suggestion feel free to drop it .