<HTML ><HEAD ><TITLE >When the clock is wrong</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.63 "><LINK REL="HOME" TITLE="The Linux System Administrator's Guide" HREF="index.html"><LINK REL="UP" TITLE="Keeping Time" HREF="c2732.html"><LINK REL="PREVIOUS" TITLE="Showing and setting time" HREF="x2757.html"><LINK REL="NEXT" TITLE="Finding Help" HREF="c2816.html"></HEAD ><BODY CLASS="SECT1" BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#840084" ALINK="#0000FF" ><DIV CLASS="NAVHEADER" ><TABLE WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="3" ALIGN="center" >The Linux System Administrator's Guide: Version 0.7</TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="bottom" ><A HREF="x2757.html" >Prev</A ></TD ><TD WIDTH="80%" ALIGN="center" VALIGN="bottom" >Chapter 13. Keeping Time</TD ><TD WIDTH="10%" ALIGN="right" VALIGN="bottom" ><A HREF="c2816.html" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="SECT1" ><H1 CLASS="SECT1" ><A NAME="AEN2804" >13.4. When the clock is wrong</A ></H1 ><P > The Linux software clock is not always accurate. It is kept running by a periodic <I CLASS="GLOSSTERM" >timer interrupt</I > generated by PC hardware. If the system has too many processes running, it may take too long to service the timer interrupt, and the software clock starts slipping behind. The hardware clock runs independently and is usually more accurate. If you boot your computer often (as is the case for most systems that aren't servers), it will usually keep fairly accurate time. </P ><P > If you need to adjust the hardware clock, it is usually simplest to reboot, go into the BIOS setup screen, and do it from there. This avoids all trouble that changing system time might cause. If doing it via BIOS is not an option, set the new time with <B CLASS="COMMAND" >date</B > and <B CLASS="COMMAND" >clock</B > (in that order), but be prepared to reboot, if some part of the system starts acting funny. </P ><P > A networked computer (even if just over the modem) can check its own clock automatically, by comparing it to some other computer's time. If the other computer is known to keep very accurate time, then both computers will keep accurate time. This can be done by using the <B CLASS="COMMAND" >rdate</B > and <B CLASS="COMMAND" >netdate</B > commands. Both check the time of a remote computer (<B CLASS="COMMAND" >netdate</B > can handle several remote computers), and set the local computer's time to that. By running one these commands regularly, your computer will keep as accurate time as the remote computer. </P ><P > XXX say something intelligent about NTP </P ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="x2757.html" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="index.html" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="c2816.html" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >Showing and setting time</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="c2732.html" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >Finding Help</TD ></TR ></TABLE ></DIV ></BODY ></HTML >