Unlike many Ancestry databases, there is only minimal explanation on the search page of the 1864 Census for Re-Organizing the Georgia Militia. However, it does say that the names were extracted from a book of indexed entries. Books have introductions and opening pages with useful information.
Use the "Browse this collection" function at the right of the database's search page, and review image 3:
...1864 CENSUS FOR RE-ORGANIZING THE GEORGIA MILITIA
[from the act mandating the census:]
...The parts of the Militia re-organization
act of December 14, 1863, which relate specifically to these records
are as follows:
SECTION II. Be it further enacted, that within four
days after being notified of his appointment, it shall be the duty of
each Aid-de-Camp to commence the enrollment of all free white males
resident in his District, who are or shall be of the age of sixteen
years, and not over sixty years, or who may come to reside with the
Distict, except those who shall actually be in the Army or Navy of the
Confederate States, or in the State service. ...
SECTION III. ...it
shall be the duty of the Aid-de-Camp, within ten days after the
enrollment has been completed in his district, to forward complete
lists, in duplicate, ...to the Adjutant and Inspector General, and to
file with the Clerk of the Superior Court in each county ...
The lists within the book, abstracted from microfilm of typed transcripts and sorted alphabetically, do not include any header information other than the county and district. What you need are the original lists, which presumably would include the names of the Aides-de-Camp and their appointment dates, and the date of enrollment completion or the date that the copy was received.
The Table of Contents ends with the "Index to Enrolling Officer & Aid de Camp," but there are no dates associated with these names. The Introduction mentions the Georgia Department of Archives & History, Atlanta GA, as the repository for the records, but does not specify whether only the transcripts used by the author are meant or if the original handwritten records might also be available in that facility.
The above repository is now the Georgia Archives, in Morrow GA. And the original census images ARE available on-line, at the Georgia Archives, University System of Georgia as "Militia Enrollment Lists, 1864". They aren't indexed by name, but are organized by county and district. In the few pages I checked, they seem to be using Dec 1863 as a reference date. This may not be true in all districts.