由于我的DAPP需要调用时间，结果发现时间显示的是Pacific Daylight Time，即太平洋夏季时间。所以需要更改Ubuntu的默认时区为北京时间。
一、首先输入指令：
date
可以看到现在显示的时间。

lqr@ubuntu:~$ date
Tue Mar 31 00:13:16 PDT 2020

二、输入指令 sudo tzselect 选择时区：

lqr@ubuntu:~$ sudo tzselect
[sudo] password for lqr:
Please identify a location so that time zone rules can be set correctly.
Please select a continent, ocean, "coord", or "TZ".
1) Africa
2) Americas
3) Antarctica
4) Asia
5) Atlantic Ocean
6) Australia
7) Europe
8) Indian Ocean
9) Pacific Ocean
10) coord - I want to use geographical coordinates.
11) TZ - I want to specify the time zone using the Posix TZ format.
\#? 4

Please select a country whose clocks agree with yours.
1) Afghanistan          18) Israel            35) Palestine
2) Armenia          19) Japan            36) Philippines
3) Azerbaijan          20) Jordan            37) Qatar
4) Bahrain          21) Kazakhstan        38) Russia
5) Bangladesh          22) Korea (North)        39) Saudi Arabia
6) Bhutan          23) Korea (South)        40) Singapore
7) Brunei          24) Kuwait            41) Sri Lanka
8) Cambodia          25) Kyrgyzstan        42) Syria
9) China          26) Laos            43) Taiwan
10) Cyprus          27) Lebanon            44) Tajikistan
11) East Timor          28) Macau            45) Thailand
12) Georgia          29) Malaysia            46) Turkmenistan
13) Hong Kong          30) Mongolia            47) United Arab Emirates
14) India          31) Myanmar (Burma)        48) Uzbekistan
15) Indonesia          32) Nepal            49) Vietnam
16) Iran          33) Oman            50) Yemen
17) Iraq          34) Pakistan
\#? 9

Please select one of the following time zone regions.
1) Beijing Time
2) Xinjiang Time
\#? 1

The following information has been given:
    China
    Beijing Time

Therefore TZ='Asia/Shanghai' will be used.
Local time is now:    Tue Mar 31 15:56:29 CST 2020.
Universal Time is now:    Tue Mar 31 07:56:29 UTC 2020.

Is the above information OK?
1) Yes
2) No
\#? 1

You can make this change permanent for yourself by appending the line
    TZ='Asia/Shanghai'; export TZ
to the file '.profile' in your home directory; then log out and log in again.

Here is that TZ value again, this time on standard output so that you
can use the /usr/bin/tzselect command in shell scripts:
Asia/Shanghai

三、根据提示再输入一遍指令：TZ='Asia/Shanghai'; export TZ

lqr@ubuntu:~$ TZ='Asia/Shanghai'; export TZ

四、再输入dat验证一下：

lqr@ubuntu:~$ date

Tue Mar 31 15:57:16 CST 2020
lqr@ubuntu:~$

显示已经设置成功！！！