Captain Promotion (and IRR info)

Received a memo in the mail yesterday from HRC St. Louis informing me that on my 5-year time-in-grade anniversary (in a couple of weeks) as a 1LT, I will be promoted to CPT. The referenced AR is AR 135-155. It looks like my previous information that suggested that IRR officers were automatically promoted when they reached their maximum time in grade for their current rank was correct.