系统时间
系统时间，由系统时钟支持，在 Unix 系统中用于跟踪时间。它可以由板载硬件时钟或外部时间服务器设置.
前言
软件时钟 vs 硬件时钟
The system clock, provided by the kernel, is implemented as a simple count of the number of ticks that have transpired since the 1 January 1970 00:00:00 UTC. This is called Unix time.
The hardware clock (also known as real-time clock or RTC) is typically a component on the mainboard. It runs non-stop — even when the computer's power supply is cut off.
UTC 时间 vs 本地时间
System time is always set to localtime as determined by the user's time zone, taking daylight saving time (DST) into account.
The time for hardware clock can be modified to represent localtime or Coordinated Universal Time (UTC). UTC is preferred because it is independent from the time zone and DST. Exceptions can be made when using a dual-boot system — when the other operating system is not configured to interpret the hardware clock as UTC, such as Windows (this can be changed — see section Dual booting with Windows).
配置
时区
为了确保时间正确，请选择正确的时区以便系统知道它所在的位置。
OpenRC
systemd
systemd comes with the timedatectl command to manage the time zone:
要检查当前区域：
user $timedatectl列出可用区域：
user $timedatectl list-timezones更改时区，例如德国：
root #timedatectl set-timezone Europe/BerlinLC_TIME
This environment variable defines formatting of dates and times. For more details see The GNU C Library
系统时钟
Typically the system clock time is set up by the hardware clock on boot. Alternatively it is possible to manually set the system clock or use a network time server.
date 命令可用于管理系统时钟时间：
要检查当前的软件时钟时间：
user $date设置系统时钟，例如 2016 年 5 月 6 日 12:34：
root #date 050612342016时间服务器
systemd
systemd 带有 timedatectl 命令来管理系统时钟时间：
要检查当前的软件时钟时间：
user $timedatectl设置系统时钟：
root #timedatectl set-time "2012-12-17 12:30:59"硬件时钟
要启用硬件时钟，必须激活以下内核选项：
Device Drivers --->
[*] Real Time Clock --->
[ ] Set system time from RTC on startup and resume
[ ] Set the RTC time based on NTP synchronization
[*] /sys/class/rtc/rtcN (sysfs)
[*] /proc/driver/rtc (procfs for rtc0)
[*] /dev/rtcN (character devices)
<*> PC-style 'CMOS'
在运行时，检查当前硬件时钟时间：
root #hwclock --show将硬件时钟设置为当前系统时钟：
root #hwclock --systohc同步硬件时钟和系统时间
Typically the hardware clock is used to setup the system clock on boot. This can be done by the kernel itself or by a boot service (init script). Also on shutdown the kernel or a service can write the software clock to the hardware clock. This aids the system in having the correct time on boot.
内核方法
On a sufficiently modern kernel (3.9 or newer), Linux can be configured to handle setting the system time automatically. To do so, enable the Set system time from RTC on startup and resume (CONFIG_RTC_HCTOSYS) and Set the RTC time based on NTP synchronization (CONFIG_RTC_SYSTOHC) kernel options:
Device Drivers --->
[*] Real Time Clock --->
[*] Set system time from RTC on startup and resume
(rtc0) RTC used to set the system time
[*] Set the RTC time based on NTP synchronization
(rtc0) RTC used to synchronize NTP adjustment
[*] /sys/class/rtc/rtcN (sysfs)
[*] /proc/driver/rtc (procfs for rtc0)
[*] /dev/rtcN (character devices)
<*> PC-style 'CMOS'
The Set the RTC time based on NTP synchronization kernel option is currently supported by chrony[1], NTP and OpenNTPD since version 5.9p1[2].
To check if the hardware time is updated, install net-misc/adjtimex and run:
root #adjtimex --print | grep statusBit 6 of the reported number should be unset (0). More information in hwclock man pages (search '11 minute mode').
OpenRC
When using OpenRC the hwclock init script can set the system clock on boot and sync system time to the hardware clock on shutdown. The service is enabled by default and should be disabled in favor of the above mentioned in-kernel method. The hwclock script should not be run when using the kernel's real time clock.
root #rc-update delete hwclock bootIt could however happen hwclock being started as dependency of another rc-service, e.g. sysklogd. In this case the rc-service osclock should be added to the same runlevel as the dependent rc-service.
If however there is a need for using the OpenRC, set both clock_hctosys and clock_systohc to YES in /etc/conf.d/hwclock. By default the service is configured for UTC time standard. To change to localtime add clock="local".
/etc/conf.d/hwclockAdding hardware clock syncclock_hctosys="YES"
clock_systohc="YES"
# clock="local"
Restart the hwclock service and have the hardware clock init script run on system boot:
root #rc-service hwclock restart
root #rc-update add hwclock boot
systemd
systemd can be used to set the system clock on boot. Use timedatectl to manage the hardware clock:
要检查当前硬件时钟时间：
user $timedatectl | grep "RTC time"将硬件时钟设置为当前系统时钟（UTC 时间标准）：
root #timedatectl set-local-rtc 0将硬件时钟设置为当前系统时钟（本地时间标准）：
root #timedatectl set-local-rtc 1故障排除
Windows 双启动
Systems that dual boot with another operating system, such as Windows, generally have a struggle over the hardware clock. To make Windows not adjust the hardware clock back to local time, add the following registry entry.
For 64-bit Windows, open regedit then browse to HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\TimeZoneInformation. Create a new QWORD entry called RealTimeIsUniversal, then set its value to 1. Reboot the system. The clock should now be in UTC time. For 32-bit Windows, follow the 64-bit instructions except use DWORD instead of QWORD.
另请参阅
- Network Time Protocol — is used to synchronize the system time with other devices over the network.
- Ntp — is a suite of tools utilizing Network Time Protocol. Their purpose is to keep the system clock in time.
- Chrony — a versatile implementation of the Network Time Protocol (NTP).
- OpenNTPD — a lightweight NTP server ported from OpenBSD.
外部资源
- https://lifehacker.com/5742148/fix-windows-clock-issues-when-dual-booting-with-os-x - Dual booting with MS Windows, set RealTimeIsUniversal. Also tested with Windows 10.
- http://tldp.org/HOWTO/Clock-2.html - The Clock Mini-HOWTO.
参考
- ↑ Comparison of NTP implementations, chrony, March 1st, 2017. Retrieved on July 11th, 2017.
- ↑ OpenNTPD. OpenNTPD 5.9p1, OpenNTPD, March 29th, 2016. Retrieved on July 11th, 2017.