Is clearly labelled as S/N : HTxxxxxxxx just below the IMEI number

Example :
Your code can be HT32WWxxxxx
the format is like HT[year][month][day].

HT - Hsinchu, Taiwan where the HTC One is manufactured.
3 - 2013 (year)
2 - February (2nd month)
W - 28 ( as W is the 28 the character- refer following list and Z is the 31st day)
1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F, G, H, J, K, L, M, N, P, R, S, T, V, W, X, Y, Z

that means the phone was manufactured on 28th February,2013 in Hsinchu of Taiwan.

Just ask seller for the first 3 digits after HT/or SH and you will know the manufacturing date
Apparently Nov n after batch is free from this problem

Read my words " Apparently ....". You should know it better than any of us

You can easily test it yourself. Try taking low light shots especially when phone is warm up