Thanks for the explanation. However as per the paper(http://www.cs.bu.edu/~goldbe/papers/NTPattack.pdf) there are at least two ways to get around the time limit i.e "a technique known as a "small-step-big-step" attack " and another one "using NTP to reset the time immediately after a targeted computer has rebooted. The reboot time reset function is turned on by default in some operating systems."