# Clock

> A Linux system actually has two clocks: One is the battery powered "Real Time Clock" (also known as the "RTC", "CMOS clock", or "Hardware clock") which keeps track of time when the system is turned off but is not used when the system is running. The other is the "system clock" (sometimes called the "kernel clock" or "software clock") which is a software counter based on the timer interrupt. It does not exist when the system is not running, so it has to be initialized from the RTC (or some other time source) at boot time. References to "the clock" in the ntpd documentation refer to the system clock, not the RTC. [The Clock Mini-HOWTO](http://tldp.org/HOWTO/Clock.html)

## Kernel Integration

### Kernel Display Message

```bash
    root@edison:~# dmesg | grep -i clock
    [    0.190320] SDIO bus = 1, name = bcm43xx_clk_vmmc, ref_clock = 26000000, addr =0x401
    [    0.208133] PTP clock support registered
    [    0.240814] hsu core clock 38 M
    [    0.242400] Switching to clocksource refined-jiffies
    [    1.534698] Switching to clocksource tsc
    [    2.076269] rtc_cmos rtc_cmos: setting system clock to 2000-01-01 00:00:09 UTC (946684809)
```

## Applications / Libraries

### Setup

#### Opkg

```bash
    root@edison:~# opkg install ntp
    Installing ntp (4.2.6p5-r6.0) on root.
    Downloading http://repo.opkg.net/edison/repo/core2-32/ntp_4.2.6p5-r6.0_core2-32.ipk.
    ntp: unsatisfied recommendation for iana-etc
    Installing ntp-tickadj (4.2.6p5-r6.0) on root.
    Downloading http://repo.opkg.net/edison/repo/core2-32/ntp-tickadj_4.2.6p5-r6.0_core2-32.ipk.
    Installing libbsd0 (0.7.0-r0) on root.
    Downloading http://repo.opkg.net/edison/repo/core2-32/libbsd0_0.7.0-r0_core2-32.ipk.
    Running useradd commands...
    NOTE: Performing useradd with [ --system --home-dir /var/lib/ntp                        --no-create-home                 y
    Configuring libbsd0.
    Configuring ntp-tickadj.
    Configuring ntp.
    ln -s '/lib/systemd/system/ntpd.service' '/etc/systemd/system/multi-user.target.wants/ntpd.service'
    root@edison:~# opkg install ntpdate
    root@edison:~# opkg install ntpdate
    Installing ntpdate (4.2.6p5-r6.0) on root.
    Downloading http://repo.opkg.net/edison/repo/core2-32/ntpdate_4.2.6p5-r6.0_core2-32.ipk.
    Configuring ntpdate.
    adding crontab
    ln -s '/lib/systemd/system/ntpdate.service' '/etc/systemd/system/multi-user.target.wants/ntpdate.service'
```

### Programs

#### Network Time Protocol Client

```bash
root@edison:~# opkg install ntp
Installing ntp (4.2.6p5-r6.0) on root.
Downloading http://repo.opkg.net/edison/repo/core2-32/ntp_4.2.6p5-r6.0_core2-32.ipk.
ntp: unsatisfied recommendation for iana-etc
Installing ntp-tickadj (4.2.6p5-r6.0) on root.
Downloading http://repo.opkg.net/edison/repo/core2-32/ntp-tickadj_4.2.6p5-r6.0_core2-32.ipk.
Installing libbsd0 (0.7.0-r0) on root.
Downloading http://repo.opkg.net/edison/repo/core2-32/libbsd0_0.7.0-r0_core2-32.ipk.
Running useradd commands...
NOTE: Performing useradd with [ --system --home-dir /var/lib/ntp                        --no-create-hoy
Configuring libbsd0.
Configuring ntp-tickadj.
Configuring ntp.
ln -s '/lib/systemd/system/ntpd.service' '/etc/systemd/system/multi-user.target.wants/ntpd.service'
root@edison:~#
```

```bash
root@edison:~# opkg install ntpdate
Installing ntpdate (4.2.6p5-r6.0) on root.
Downloading http://repo.opkg.net/edison/repo/core2-32/ntpdate_4.2.6p5-r6.0_core2-32.ipk.
Configuring ntpdate.
adding crontab
ln -s '/lib/systemd/system/ntpdate.service' '/etc/systemd/system/multi-user.target.wants/ntpdate.servi'
```

```bash
root@edison:~# ntpdate -s time.nist.gov
root@edison:~# date
Mon Mar 21 20:14:51 UTC 2016
```


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