I have the following code for what you ask for, because I also use it and I have asked about it in the forum, since I couldn't find the original message I'll leave it here.
Create a custom column of name days
Column generated from other columns
In template paste this code adding the name of your columns of the start and end dates of readings
Code:
program: format_number(days_between(raw_field('#your date column ended'), raw_field('#your date column started')),"{0:5.0f}")
and it will look like the picture