In "Maryland Militia in the War of 1812" (volume 3), Lt. Col. William C. Miller was commissioned as commander of the 30th Regiment, Cecil County, on March 14, 1810.
In Volume 8, it states that men stood guard at Ft. McHenry for 3 weeks under the command of "Lieut. Colonel William C. Miller of Cecil County," and "they were discharged on 29 June, 1813."
It further states that "In 1813 a specially formed first regiment, 'in the service of the United States' at Baltimore was placed under the command of Lieut. Colonel William C. Miller of Cecil County." I have an ancestor who served in the same regiment at the same time. You can request his service record from the National Archives. Specify "1st Regiment (Miller's) Maryland Militia (War of 1812)."