不记下来，难免会忘记~

- 阅读剩余部分 -

Install 'net-tools' and 'wget' packages on CentOS 7:

yum install net-tools wget

https://openvpn.net/index.php/access-server/download-openvpn-as-sw.html

Type wget followed by the pasted URL:

wget <paste copied url>

Install downloaded package on RedHat/CentOS/Fedora system:

rpm -Uvh openvpn-as-2.1.12-CentOS7.x86_64.rpm

The installation process should then commence and finish. The output may look like this:

The Access Server has been successfully installed in /usr/local/openvpn_as Configuration log file has been written to /usr/local/openvpn_as/init.log Please enter "passwd openvpn" to set the initial administrative password, then login as "openvpn" to continue configuration here: https://192.168.70.222:943/admin To reconfigure manually, use the /usr/local/openvpn_as/bin/ovpn-init tool. Access Server web UIs are available here: Admin UI: https://192.168.47.222:943/admin Client UI: https://192.168.47.222:943/

The Access Server tries to adapt itself to the network configuration it finds. But if you have a complex network setup you may need to run the ovpn-init tool to reconfigure to listen to another network interface. It's worth mentioning that the ovpn-init tool can be used to wipe all OpenVPN Access Server configuration with the --force option in case you make a mistake during initial setup. Don't use the --force option on an existing installation unless you can live with losing all configuration and certificates and want to start over. Also, the program will ask for a license key, but you do not have to enter one. If no valid license key is found it will just assume a demonstration mode where all functions work but you're limited to 2 simultaneous VPN tunnel connections.
Finishing configuration and using the product
Once the program is installed it will automatically configure itself with some standard settings. The installation process will also tell you where to find the client web service, which is the web based GUI that you can use to log on and connect to the Access Server, and where to find the admin web service, which is where you can log on as an administrative user and manage the configuration, certificate, users, etcetera, in the web based GUI. Usually the client UI is at the address of your server, for example https://192.168.70.222/. The admin UI is usually at the /admin/ address, for example https://192.168.70.222/admin/. Please note that the web services by default actually run on port TCP 943, so you can visit them at https://192.168.70.222:943/ and https://192.168.70.222:943/ as well. The OpenVPN TCP daemon that runs on TCP port 443 redirects incoming browser requests so that it is slightly easier for users to open the web interface.

Initially a single administrative user is added to the system. But it has no password set and therefore cannot be used yet. To use it a password must be set first:

passwd openvpn

You can now point your web browser at the admin UI web interface. Because the Access Server comes with a self-signed SSL certificate to begin with, you will receive a warning in the browser like "Invalid certificate" or "Cannot verify identity of the server". You will have to confirm that you wish to continue to the web interface. You will then see the login screen and you can then enter the username openvpn and the password you have just set with the "passwd openvpn" command.

https://docs.openvpn.net/getting-started/installing-openvpn-access-server-on-a-linux-operating-system/

在平常写项目的时候，不可避免的会用到时间计算。
如果只是简单的计算的话，只需要将时间字符串转换为时间戳然后对比即可。
但是如果需要计算具体的年月日时分秒的话，可以用日期对象来计算。

普通简单计算

// 时间1
$date1 = strtotime('2018-10-01'); // 1538352000
// 时间2
$date2 = strtotime('2018-11-01'); // 1541030400
// 时间2 比 时间1多多少秒
$diff = $date2 - $date1; // 2678400

使用时间对接来计算

http://php.net/manual/zh/datetime.diff.php

// 出生日期
$birthday = new \DateTime('1996-10-11');
// 当前时间
$now = new \DateTime();
// 计算出生日期和当前时间的时间查
$interval = $birthday->diff($now);
var_dump($interval);

打印出来是以下信息

object(DateInterval)[18]
  public 'y' => int 21 // 年
  public 'm' => int 6 // 月
  public 'd' => int 0 // 日
  public 'h' => int 17 // 时
  public 'i' => int 16 // 分
  public 's' => int 2 // 秒
  public 'weekday' => int 0
  public 'weekday_behavior' => int 0
  public 'first_last_day_of' => int 0
  public 'invert' => int 0
  public 'days' => int 7852 // 天数
  public 'special_type' => int 0
  public 'special_amount' => int 0
  public 'have_weekday_relative' => int 0
  public 'have_special_relative' => int 0

看上面的打印信息，就能看到很具体的时间了，而不用另外去计算
也就是说，生日至今，年龄是：21岁6个月17小时16分2秒，7852天

系统名称后面带个N的，代表移除了某些功能，例如：Windows Media Player, Groove Music, Movies & TV, Voice Recorder, Skype等
https://support.microsoft.com/en-us/help/3010081/media-feature-pack-for-windows-10-n-and-windows-10-kn-editions

Windows Server 版本 1709

Windows Server 2016

Windows 10 版本 1709

Windows 10

Windows Server 2012 R2 and Windows 8.1

Windows Server 2012 and Windows 8

Windows 7 and Windows Server 2008 R2

Windows Vista and Windows Server 2008

Office 2016

Office 2013

Office 2010

参考资料：

Server/Windows：https://docs.microsoft.com/zh-cn/windows-server/get-started/kmsclientkeys