After completing 14 home-study Army Extension Courses, Reagan enlisted in the Army Enlisted Reserve and was commissioned a second lieutenant in the Officers' Reserve Corps of the Cavalry on May 25, 1937.  On April 18, 1942, Reagan was ordered to active duty for the first time. Due to his poor eyesight, he was classified for limited service only, which excluded him from serving overseas. His first assignment was at the San Francisco Port of Embarkation at Fort Mason, California, as a liaison officer of the Port and Transportation Office. Upon the approval of the Army Air Forces (AAF), he applied for a transfer from the cavalry to the AAF on May 15, 1942, and was assigned to AAF Public Relations and subsequently to the First Motion Picture Unit (officially, the "18th Army Air Force Base Unit") in Culver City, California. On January 14, 1943, he was promoted to first lieutenant and was sent to the Provisional Task Force Show Unit of This Is the Army at Burbank, California. He returned to the First Motion Picture Unit after completing this duty and was promoted to captain on July 22, 1943.  In January 1944, Reagan was ordered to temporary duty in New York City to participate in the opening of the Sixth War Loan Drive, which campaigned for the purchase of war bonds. He was reassigned to the First Motion Picture Unit on November 14, 1944, where he remained until the end of World War II. He was recommended for promotion to major on February 2, 1945, but this recommendation was disapproved on July 17 of that year. While with the First Motion Picture Unit in 1945, he was indirectly involved in discovering actress Marilyn Monroe. He returned to Fort MacArthur, California, where he was separated from active duty on December 9, 1945. By the end of the war, his units had produced some 400 training films for the AAF.

Answer this question "When did Reagan join the military?" by extracting the answer from the text above.
Reagan enlisted in the Army Enlisted Reserve and was commissioned a second lieutenant in the Officers' Reserve Corps of the Cavalry on May 25, 1937.