Could have technically happened anytime from apr 4, 2169 to apr 3, 2170, he just needed to be 5 years old. Most likely in the late spring to early fall, since the trees shown still had their leaves. AFO presumedly found (re: kidnapped) him some time afterwards, but in between that, he was kinda just. wandering homeless for some time. AFO probably didn't find him any later than early winter, because i doubt that Tomura could have survived that.