To address some of the recent DST postings... Clocks with hard-coded USA DST rules will be off by an hour for the three weeks from 11-Mar to 1-Apr-2007. Given how long it's been since the DST rules changed you can see why someone in a cheap or weak moment would design a clock with hardcoded rules. Still, a bad design.
WWVB radio-controlled "atomic clocks", on the other hand, do not have hardcoded rules; instead they switch to DST based on command from Boulder. This is why many of us saw our RC clocks/watches do the right thing this morning. This is a good design. But a big problem remains. The WWVB subcode has only the ability to give 1 UTC day of advanced warning of a DST change. So if your clock happened to have poor reception yesterday it still doesn't know of the DST change and will remain in error by an hour until it ever gets good reception, which could be day(s) later. The problem is compounded by the fact that most RC clocks only enable reception late in the evening (e.g., starting at 11 PM), that the DST switch occurs at 2 AM local time, and that most of the USA is 5 to 8 hours left of Greenwich. These three factors make the window for DST notification much smaller than one day. And the result is that every time a DST change occurs there are tens of thousands of RC clocks that get it wrong (by not getting it at all). It's all a little embarrassing since these clocks are often advertised to be accurate to a millionth of a second, etc. Further embarrassing is that NTP, the great internet clock is so academically pure, and that GPS, the great clock in the sky, is so globally available, that neither dare taint themselves with the geographical and political mess of timezones or DST. A solution would be to carve out a few more DST bits in the WWVB subcode. So instead of giving a few hours of notice an RC clock would see, for example, a 7- or 15-day binary countdown to the DST event. That way, poor reception the night before DST, or even a couple of nights before, would not make the clocks fail at 2 AM Sunday. Don't hold your breath waiting for a fix; but at least you better understand the problem now. Actually, the solution may be that more and more people are using cell phones instead of clocks/watches/computers to get accurate local time... /tvb http://www.LeapSecond.com Links: NIST Radio Station WWVB http://tf.nist.gov/stations/wwvb.htm WWVB Time Code Format http://tf.nist.gov/stations/wwvbtimecode.htm WWVB Radio Controlled Clocks http://tf.nist.gov/stations/radioclocks.htm http://tf.nist.gov/general/pdf/1976.pdf http://tf.nist.gov/general/pdf/1383.pdf Decoding WWVB from a Sony atomic... http://www.leapsecond.com/pages/sony-wwvb/ WWVB Subcode Test Generator - wwvb2 http://www.leapsecond.com/notes/wwvb2.htm _______________________________________________ time-nuts mailing list [email protected] https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
