This counter can be found under My Information -> History tab and shows how many mobs you have killed in 1 day. Now I've always thought it weird that the daily reset time for this counter is 16:58 but it got even weirder when ppl in guild chat told me their reset time is different from mine. Based on example times ppl brought up time-zone can be ruled out as a reason for the differences.

So what is it based on?