Drew Arthur wrote: >We would like to understand the true time setting variables that are used by >a Linux OS and zLinux guests running under VM.
Linux (like other Unixes) maintains its internal system time always in UTC (GMT). This is used for all permanent records like file time stamps. When displaying time to the user, Linux applications usually convert the system time from UTC to the local time format. This involves applying the correct offset depending on the selected time zone; in particular, all applicable daylight savings time rules (and things like leap seconds etc.) are automatically applied in this conversion step. There's nothing you need to do manually when daylight savings time comes into effect. The other question is how the master system clock gets *set*. During normal operation, the system increments its master clock using a periodic clock tick interrupt. You still need to get an initial value at system boot time, and also you'll want to reset the clock from time to time due to inaccuracies creeping in. For that, there are basically three options. You can set the clock manually, you can consult a hardware clock, or you can set up NTP. When setting the clock manually, you enter the current *local* time, which is automatically converted back to UTC in order to set the system clock. When using a hardware clock on a PC, there are two options: the hardware clock can be set to local time (which you usually do if you share the PC with Windows) or to UTC (which you do if Linux runs as the only operating system). Therefore, you need to tell the hwclock utility in which of those two modes your hardware clock is operating (note that the Linux system clock itself is *always* UTC). *This* is what the SuSE installation asks at startup (note that this question is pointless on S/390, see below). On S/390, we use the TOD clock as 'hardware clock'. However, this works significantly different from a PC in that: - we don't support the hwclock utility, but use the TOD clock only once at boot to initialize the system clock - we assume that the TOD clock is always set to UTC (when running under VM you may need to set up guest parameters accordingly) Finally, when using NTP everything works automatically, and the clock is always set correctly; if you can do that, this is certainly the preferred method. Bye, Ulrich -- Dr. Ulrich Weigand [EMAIL PROTECTED]
