DOS spans two days

Encinitas, CA
Best answers
I'm coding a procedure that started late on one day, and ended the next morning. The handwritten and dictated reports are dated on the second day. What DOS should I use? I haven't been able to locate any resources (CMS preferably) to back up my decision. Any advice, and references if possible? Thanks!
I'm with Tessa, we've always used the date the procedure started as our date of service. The only time that I have done otherwise was in anesthesia, but our billing system required us to enter as two charge lines when the procedure cross the midnight hour.