Revising duration so it doesn't count first day


I want the range of days from Monday the 1st to Monday the 21st to read 21 days; yet it reads "22 days" in the duration column.  This is because day 1 is the first day, when I need "day 1" to be the following date from the first day.  Is there any easy way to fix this?  In my job I can never count the first day as day one as it's against policy in how we count days. 


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