How do i write my expression for finding days between 2 separate days?

[Robert_Elgstrand]

Hi. 

I've been trying to use expressions to find out how many days there are between two specific dates added to an object in sharepoint. ! can't get it to work, really frustrating!!!

Lets say my startdate is 2018-12-18 and my end date is 2018-12-21 that's 4 days, but how do i use an expression (or function, whatever it's called) to find the number of days between dates?  

[#2]

Have you checked this answer?
https://powerusers.microsoft.com/t5/Building-Flows/Compare-dates/td-p/54802

[#3]

I was able to solve it in another way, which works equally well. But thanks, I'll go in there and read it all.