The tombstone dates are "C 1277.1 TO 1313.7" Mitchell may have been insulting Kirk by calling him a one-year-old and getting his initial wrong
Yeah. If we think 1000 SD = 1 yr, then those dates would mean that Kirk's birthday and day of death would fall on the same year (the fourth digit from the right) but naturally on a different decade (the missing fifth digit). This would mean that since he's 34 around SD 3478 ("The Deadly Years"), he'd be about 31-32 in SD 1313, and thus 21-22, 11-12 or 1-2 in SD 1277 - the "system" would not allow him to be born on SD 1277 or any of its decade variants.
On the other hand, like Robert Comsol
implies, Mitchell could be saying that Kirk was Captain ("C") of the Enterprise
for that length of time, which is perfectly okay. Really, what else could the letter "C" mean? It's not standard tombstone symbology, now is it? But it is something Mitchell might want to remind Kirk of ("See how you failed in your life's work and ambition?") plus something that would not require the decade digit.