Thank you for the reply. I am not trying to calculate the number of days.
For example, for any given non-standard month (21st to 20th), I need to know how may days I have remaining in the non-standard month. For instance, today, the 19th, I have one day remaining in the NS month.
So, when I'm calculating the total time spent per worker, I need to be looking at all the work that they have completed between the 21st of last month and today.
Hopefully, that makes it a little clearer.