How to modify and synchronize system time in Linux

Time modification
date month day hour minute year. second
date -s can directly set the system time
For example, the command to set the system time to June 10, 1996 is as follows.
#date -s 06/10/96
The command to set the system time to 1:12:00 pm is as follows.
#date -s 13:12:00
Time synchronization
1. First of all, you need to understand crontab, a task planning tool in Linux
crontab can regularly perform the actions you want to do
Use it directly Crontab command editing
crontab -u //Set the cron service of a certain user. Generally, the root user needs this parameter when executing this command.
crontab -l //List the details of a certain user's cron service.
crontab -r //Delete a user's cron service
crontab -e //Edit a user's cron service
Generally use crontab -e to edit a planned action, the editing mode is the same as the vi editor Same
The basic editing format in crontab is as follows
*　## The 2nd column represents hours 1 to 23 (0 represents 0 o'clock)
The 3rd column represents the date 1 to 31
The 4th column represents the month 1 to 12
The 5th column identifies the day of the week 0 to 6 (0 means Sunday)
Command to run in column 6
Some examples of crontab files:
30 21 * * * /etc/init.d/smb restart
The above example means every night Restart smb at 21:30.
45 4 1,10,22 * * /etc/init.d/smb restart
The above example indicates that smb will be restarted at 4:45 on the 1st, 10th, and 22nd of each month.
10 1 * * 6,0 /etc/init.d/smb restart
The above example indicates that smb will be restarted at 1:10 every Saturday and Sunday.
0,30 18-23 * * * /etc/init.d/smb restart
The above example indicates that smb will be restarted every 30 minutes between 18:00 and 23:00 every day.
0 23 * * 6 /etc/init.d/smb restart
The above example indicates that smb is restarted at 11:00 pm every Saturday.
* */1 * * * /etc/init.d/smb restart
Restart smb every hour
* 23-7/1 * * * /etc/init.d/smb restart
Between 11 pm and 7 am, restart smb every hour
0 11 4 * mon-wed /etc/init.d/smb restart
On the 4th of every month and every Monday to Wednesday Restart smb at 11 o'clock
0 4 1 jan */etc/init.d/smb restart
Restart smb at 4 o'clock on January 1st
2. Find a network time server, such as some national time service centers
Microsoft Corporation Time Service Host (United States)
time.windows.com
Taiwan Police University Time Service Center (Taiwan)
asia.pool.ntp.org
Chinese Academy of Sciences Time Service Center (Xi’an)
210.72.145.44
Netcom Time Service Center (Beijing)
219.158.14.130
3. How do our computers synchronize the time of the time service center
ntpdate asia.pool.ntp.org
This command above It is the time to synchronize the timing center.
If you want the system to synchronize at 23:00 every day, you can use the crontab setting plan synchronization mentioned above.
The following command
#crontab -e
Enter the crontab editing mode , the usage method is the same as vi
Enter 0 23 * * * ntpdate asia.pool.ntp.org >> /var/log/ntpdate.log
Save and exit
This will complete your system Go to asia.pool.ntp.org at 23:00 every day to synchronize the time, and put the synchronized log into
/var/log/ntpdate.log

The following will add the system time settings
date -s can directly set the system time
For example, the command to set the system time to June 10, 1996 is as follows.
#date -s 06/10/96
The command to set the system time to 1:12:00 pm is as follows.
#date -s 13:12:00
Note that what is mentioned here is the system time, which is maintained by the operating system of Linux.
When the system starts, the Linux operating system reads the time from CMOS into the system time variable. Later, the time can be modified by modifying the system time. In order to keep the system time consistent with the CMOS time, Linux writes the system time to the CMOS every once in a while. Since the synchronization is performed at regular intervals (about 11 minutes), if we restart the machine immediately after executing date -s, the modification time may not be written to CMOS, which is the cause of the problem. If you want to ensure that the modification takes effect, you can execute the following command.
#clock -w
This command forces the system time to be written to CMOS

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