Customer Service Representative

I work a 9-5 job. I punch in early evey day and punch out late every day but only get paid for a straight 8 hours. Accoding to the DOL, if my employer chooses to "round down" when calculating OT, then he must practice this both ways. In other words, if I punch in at 8:50am and punch out at 5:07pm, then I am entitled to the 17 minutes as overtime. I have asked for proof of this law in writing, but to no avail. They have sent me everything covering OT calculating, but nothing substantial to bring to my employer and request an audit of my personnel file to collect back wages. How can I get the "law" in writing regarding OT minutes (not hours) to bring to my employers attention without having to file a claim against him?