UptoDate recommends that you cite the UpToDate topic as a chapter in a book titled UpToDate. There are no page numbers to cite, and the publication year for any topic should be the current year.
The example provided by UptoDate:
Marion, DW. Diaphragmatic pacing. In: UpToDate, Post TW (Ed), UpToDate, Waltham, MA. (Accessed on November 25, 2013.)