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    Respectfully "high stratum" does not mean low stratum number this should be removed from the answer. S2 is higher than S1. For evidence look at the explanation by Prof. Mills (aka Prof NTP) "The NTP subnet operates with a hierarchy of levels, where each level is assigned a number called the stratum. Stratum 1 (primary) servers at the lowest level are directly synchronized to national time services...Stratum 2 (secondary) servers at the next higher level are synchronized to stratum 1 servers and so on." eecis.udel.edu/~mills/ntp/html/warp.html
    – dfc
    Commented Apr 25, 2014 at 12:26
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    Its better but I do not think that stratum 0 is "by definition an accurate time source." Stratum 0 simply means it is a reference clock of some sort. Just because it is an S0 clock does not mean it is properly functioning. Stratum level is about distance from reference clocks not guaranteed accuracy.
    – dfc
    Commented Apr 25, 2014 at 13:35
  • @dfc I appreciate the input as it allows me to make the answer even better. That said, I disagree with your statement that S0 does not mean it is accurate. Maybe that is so in a perfectly strict sense, but if the time given by a S0 clock is inaccurate, that clock is useless in practice as a S0 source. Ergo, S0 clocks are accurate. Or put another way, if you have an isolated set of hosts, one of which is connected to a S0 time source and all others sync against that, the time given by the S0 source is the correct time as far as those hosts are concerned. No need to worry about TAI then.
    – user
    Commented Apr 25, 2014 at 14:08