I've noticed that the clock on the city market runs slow. I've found that since it refreshes every 6 hours I can take advantage of all 4 times if I have it refresh at 9am, 3pm, 9pm and 3am. I can't always get the booster and if I did it wouldn't be very advantageous since I'd only be able to take advantage of 2 more refreshes per day. What I have seen is that the market refreshes a little later each day. This morning it's refreshing at 9:18am. In another week or so it'll be 9:30 or later eventually forcing me to pay T-cash to get it back on schedule. It'd be nice if the devs could look into that.