To change the topic from the "time_t doesn't work for everything" series of messages here is a note on about synchronizing clocks. And about the limitations of time_t, it is something we can live with. Any extensions should be extensions and not modification to time_t. Too breakable. This has nothing at all to do with Posix, but, I thought that this group might like to hear about it. Ignore if you want. If the wording seems strange at times, bear with it. This was written in a stream of consciousness mode. Note that there are many interesting algorithms described in the OS literature about clock synchronization. Dave Mills has been playing in the real world (TCP/IP connected machines) on how to synchronize clocks. He found that human error accounts for a lot of the problem (as is: "ahhh, was that supposed to be 'AM' or 'PM'?"). ---- Suppose you have N clocks that you would like to synchronize. How would you do it? The answer is "it depends". That is, it depends on how much money you want to spend and how closely synchronized you want the clocks to be. You can see that if you only want to spend one postage stamp, the best you can achieve is +/- several days (the time it takes the mailperson to deliver the letter containing a timestamp). If you cough up some bucks for two modems, you can do better. Etc. What do I mean by synchronization? The best explanation that I've come up with is: consider how the Air Force's precision flying team puts on a show. What matters to them is, first, how tight the formation is, and second, where the planes are with respect to some ground-based marker. Translating that example into clocks shows that clocks can be very close to each other (== synchronized) but still give the incorrect time (!= universal time). Sometimes synchronization is more important than correctness (distributed databases that use timestamps are an example). To get synchronization of clocks AND correctness calls for two things. A mutually agreed upon reference time source supplies the correct time. A convergence algorithm used between clocks supplies the synchronization. Here's a list of time sources that I've heard of: 0. Set clocks at "factory" I include this for completeness. Perhaps the atomic clocks are set this way. I suspect some clocks in satellites are set this way. 1. The electrical "power grid" This is the familiar 60 Hz (or 50 Hz) AC electric line. One could devise a simple counter of the frequency so that all clocks would always be in step with each other. Unfortunately there are several problems. The first is how to initially synchronize the clocks. Also, there are several (5?) different grids in the 48-state area of the US that are not in sync with each other. Plus, each power-grid master clock is only consistent when averaged over the entire day. When peak demands occur, you aren't getting the full 60 Hz per second that you would expect. I've heard that by about 6pm, the grid is lagging by up to 10 seconds. However that lost time is made up while (most) people sleep so that the alarm clock is correct at 8am. [as an aside: leave work early tomorrow and if anyone asks, just give them the above reason :-)] 2. Phone system Each line used by for phone traffic has some timing-dependencies. Major trunk lines have to synchronize. I don't know the details of how the clocks are set. I vaguely remember something about a master clock for AT'n'T located in Indiana (Illinois?) that serves as the reference clock for the entire network. Anyone know more? Problems with this is that the clock is not reachable by users not in the phone company for free (maybe one could lease a T-1 channel and count bits or watch for the 193'rd etc.). 3. Solar based I've read where early clocks were synchronized to noon by someone who watched an instrument's scale for the instant that the sun be at the highest point in the sky. A tiny filament in the instrument served as the focal point for a slit of sunlight, the instrument could only be tilted north and south so that the seasonal tilt was adjusted for. Problems are that it is tedious and labor-intensive. How to set up the solar spotting tube sounds pretty tricky to me. 4. Universal time This is the collection of atomic clocks (over 100 of 'em) around the world that maintain the time. What's good is that is set up, working, and is correct. Problem is dissemination. Unless you are close to an actual clock, there is some delay. However, it is on the order of milliseconds, not too bad. Expenses are getting a receiver for the signal. In the US, there are several frequencies for WWV. Worldwide, one can use a satellite's broadcast if you are lucky enough to be in its "footprint". The GOES satellite serves North America and some of Pan America. I don't know if there is an equivalent satellite for Europe, Africa or Asia. Using a satellite means you have to pop for a dish antenna. 5. Neutron star A spinning neutron star is very periodic. Some are more than others. One neutron star has been watched for over a decade. During that time, it has shown remarkable consistency -- on par with an atomic clock. I'd have to rummage through my notes to get its designation. Problems: besides buying and installing a radio telescope it is very easy... OK, we got a source of time. Now how do we set the clocks? A convergence algorithm can be structured one of two basic ways. The simplest is in a master/multiple-slaves organization. The harder to implement way is in a distributed fashion where each clock shares its "time" with all other clocks. One has to worry about faults (and Byzantine faults, ie, clocks that are "broken" and give different readings depending on who asks it for the time), and compensating for delay in messages between clocks. But the second way leads to some nice advantages for a data network (deadlock detection is easier, can timestamp things for local and remote use, ...) Another problem is related to how clocks move -- forward. Suppose you are going to reset your clock to a new value of time. What happens if that new value is less than the current reading of a clock? Strange things may happen if you set the clock back! Two ways around this are (1) to "stop" the clock until the clock's reading equals the new time and (2) to run the clock slower for a while. On my old pendulum clock at home, I can shorten or lengthen the pendulum if the clock is running slow or fast, respectively. One can't really change the rate of a digital clock. So, one uses the first method. For an OS that updates its clock by seeing interrupts, if the clock is too far ahead some of the updates can be skipped. If the clock is behind, double up some of the clock updates. Careful consideration needs to be done to see if anything (accounting for one!) breaks. Best to do it when no one is using the machine (but then if updates are scheduled for at night and the machine's clock is 12 hours off, strange things will happen). Berkeley's TEMPO time synchronization protocol changed how the clock is set to be better than one second resolution. [If I could find the paper now, I could tell you more. It was in a Usenix proceedings of a couple years ago.] Bob Devine