All CSA military units where mustered on the last day of even numbered months. This included a roll call, an inspection and payday (if money was on hand, which was rare).
source "Regulation of the Army of the Confederate States 1864"
A constript was a person who was drafted.
Since Pvt. Gilbert was a miller, he more that likely had an exemption until 1863. Exmemptions allowed the person to stay out of the military as long as they provided their services to their community and the CSA.

The first Confederate conscription law applied to men between 18 and 35, providing for substitution (repealed Dec. 1863) and exemptions. A revision, approved 27 Sept. 1862, raised the age to 45; 5 days later the legislators passed the expanded Exemption Act. The Conscription Act of Feb. 1864 called all men between 1 7 and 50. Conscripts accounted for one-fourth to one-third of the Confederate armies east of the Mississippi between Apr. 1864 and early 1865.
Source: "Historical Times Encyclopedia of the Civil War" Edited by Patricia L. Faust

Maybe George or someone else will jump in with more information.

Mike