The date?

What is the fictional date of The Good, The Bad, and the Ugly (1966)?

I always assumed that because General Sibley's invasion of New Mexico happened in 1862 in real history it must have happened in 1862 in The Good, The Bad, and the Ugly (1966) also.

I just watched it and the headboard of Arch Stanton in the cemetery at the end said: "Arch Stanton 3 February 1862". So Arch Stanton died on 3 February 1862 and the movie happens later in 1862 or possibly in 1863, 1864, or 1865.

When the coffin of Arch Stanton was opened a skeleton was seen. So the body was in the coffin for at least the minimum number of months or years it would take to become a skeleton. So how long should that have taken?

So how much time passes during The Good, The Bad, and the Ugly (1966)? Is it possible that the scenes with Sibley's invasion of New Mexico happen in 1862 but the last part of the movie is in 1863, 1864, or early 1865 before the war ends?

I found this statement:

The Good, The Bad, and The Ugly has grave markers in the military cemetary shown dated 1862 and 1864, while the war is still active, so the events occur in 1864 - 1865.

When the POW camp commandant is upbraiding Angel Eyes for his treatment of the Confederate prisoners, Angel Eyes responds with a comment about the treatment of Union POWs at Andersonville. Andersonville was opened in 1864. The Film is set in 1862.

In one scene, Tuco praises Lee and damns Grant out loud to the troops coming out of the desert. However, the movie takes place during Confederate invasion of New Mexico Territory in February- March of 1862, when both Lee and Grant were unknowns at this time. Lee didn't assume command of the Army of Northern Virginia until June 1862. Grant was a relative unknown when he won his first victory at Fort Donelson in February, 1862, hardly enough time for Tuco and Blondie to know who he was. That being said, in another scene, Angel Eyes mentions Confederate abuses in Andersonville Prison, which only became a prison and started accepting prisoners in February 1864.

The battle-scenes show at least two Gatling guns, forerunner to the machine gun. The Gatling gun was patented in Feb 1862 but did not see action until mid-1864 while the movie takes place early 1862. The Gatling gun first saw action in Virginia during the Petersburg siege of June 1864. The Gatling gun was not formally introduced into the U.S Army until 1866.