1. Introduction

The Real-Time-Clock (RTC) chips used on PC motherboards (and even expensive workstations) are notoriously inaccurate, usually gaining or losing a consistent amount of time each day. Linux provides a simple way to correct for this in software, which can make the clock *very* accurate, even without an external time source. But most people don't know how to set it up, for several reasons:

• It's not mentioned in most of the general "how to set up linux" documentation, and it can't be set up automatically without an external time source, so the default is not to use it.
• If you type "man clock" you may get the man page for clock(3), which is not what you want (try "man 8 clock" or "man 8 hwclock"-- some distributions search in numerical order if you don't give a section number, others search in the order specified in /etc/man.config).
• Most people don't seem to care what time it is anyway.
• Those few who do care often want to sync the system clock to an external time source, such as a network time server or radio clock. This makes the accuracy of the RTC irrelevant.

This mini-HOWTO describes the low-tech approach (which can be very accurate by itself), and provides pointers to several more sophisticated options. In most cases the documentation is well written, so I'm not going to repeat that information here, but I've included detailed instructions for the old clock(8) program at the end for anyone still running an older system.

Note

You must be logged in as "root" to run any program that affects the RTC or the system time. Keep this in mind when setting up any of the programs described here.

Note

If you run more than one OS on your machine, you should only let one of them set the CMOS clock, so they don't confuse each other. This includes the twice-a-year adjustment for Daylight Savings Time.

If you run a dual-boot system that spends a lot of time running Windows, you may want to check out some of the clock software available for that OS instead. Follow the links on the NTP website at http://www.eecis.udel.edu/~ntp/software.html. Many of the radio clocks mentioned here include software for Windows.