I know they didn’t use the names of the months or summary the numbers in years like 2011 instead of spelling the year 2011 in full, but please help me to understand what it means 3 Nephi 8:5 “in the thirty and fourth year, in the first month on the fourth day” does it mean Jan 4th, 34AD? and 3 Nephi 10:18 means Jan 31 or Dec 31?
It means after their reckoning, it was the 4th day of the 1st month in the 34th year after the sign had been given of the coming of Christ. We don’t know exactly when that time was, although the Book of Mormon does give us references at the bottom of the pages that MAY coincide with the years given in the record. Please don’t assume that their calendar matches our calendar, which was founded during Roman times, long after the Nephites/Lamanites left for the new world (and then was recalibrated in the 1600s, I believe, to adjust for some lost time).