The production date number of Sony 18650 cells is hidden in the second row of codes, so it is a little hard to find it. Usually, it is the last 5 numbers + letters.
For example: Sony US18650VTC4 2100mAh 3.7V.
The second row: C4A1XXK21G. The last 5 numbers + letters: XK21G.
1. The first letter "X" = Year (1992=A; 1993=B; ... 2014=W; 2015=X; 2016=Y, and so on...)
2. The second letter "K" = Month (JAN=A; FEB=B; ... SEP=I; OCT=J; NOV=K; DEC=L)
3. The third and fourth number "24" = Day (1st=01; 2nd=02; ... 30th=30; 31st=31)
4. The fifth letter "G" = Electrode history (A, B, C, ... X, Y, Z)
Above all, code: XK21G means the battery made in 21th. November. 2015.