title: Calculate dates by giving a default day of month using SQL Server 2008
body: <p>I need to calculate a future date by subtracting a date from another date.  The catch is the date field I am subtracting has many days of the month...i.e. 10/27/2020 or 11/30/2020.  What I need to do is, no matter what the day of the month is, I need to subtract the month as if it were the first day of the month no matter what it may be.</p>
  <p>So for example I have a date 12/31/2020  I need to subtract another date 10/23/2020 from that date but I need to default 10/23/2020 to 10/01/2020 and same for any other date I would subtract from the original date.  Any ideas?</p>