The 12th New York Infantry Regiment (3 years) is a different unit that was organized in May.

In April of 1862 legislation changed the designation of militia units to “National Guard.”

1861
April 21 Sailed from New York on steamship Baltic for Fortress Monroe, Va. under Colonel Daniel Butterfield
April 23 Arrived Fortress Monroe, then moved via the steamsip Goatzacoatcos to Annapolis and Washington, D.C. Assigned to Mansfield’s Command.
May 2 Mustered into Federal service for three months
May 23 Advanced into Virginia across the Long Bridge and to Roach’s Mills
May 24 Occupation of Arlington Heights, Va.
June 2 Returned to Washington
July 6 Ordered to join Patterson’s Army
July 7 Left Washington by rail for Hagerstown, Md. via Baltimore and Harrisburg
July 9 Marched from Hagerstown to Williamsport, forded the Potomac and continued to Martinsburg. Assigned to Third Division, 8th Brigade, Patterson’s Army. Colonel Butterfield took command of the brigade, and Lieutenent Colonel Ward took over the regiment.
July 12 Skirmish near Martinsburg
July 15 Near Bunker Hill
July 17 To Charlestown
July 21 Marched to Harpers Ferry
July 26 Four companies crossed the Shenandoah in flatboats and occupied blockhouses built by the Confederates on Loudon Heights.
July 28 Evacuated Harpers Ferry and marched to Knoxville
August 1 Moved to New York City
August 5 Mustered out of United States Service at New York City by Lieutenant Colonel Sheppard
1862
May 27 Nine companies ordered to Washington under Colonel William G. Ward
June 6 Mustered into United States service for three months and attached to 4th Brigade at Harpers Ferry
August 27 Volunteered to extend its terms of service
September 12-15 Defense of Harper’s Ferry
September 12-13 Maryland Heights
September 15 Surrender of Harper’s Ferry.
September 16 Paroled as prisoners of war
October 12 Mustered out
1863
January 11 Declared exchanged
June 18 Ordered to Harrisburg, Pa.
June 20 Mustered into United States service for thirty days under Colonel Ward and attached to General Yates’ command at Fenwick, Pa. in the 1st Brigade, Dana’s Division, Department of Susquehanna
July 20 Mustered out of United States service at New York City