Added section on system time

docs/requirements.md

@@ -1,7 +1,8 @@
 Before you run **mailcow: dockerized**, there are a few requirements that you should check:
 
 - **WARNING**: When you want to run the dockerized version on your Debian 8 (Jessie) box you should [switch to the kernel 4.9 from jessie backports](https://packages.debian.org/jessie-backports/linux-image-amd64) because there is a bug (kernel panic) with the kernel 3.16 when running docker containers with *healthchecks*!
-  Full more details read: [github.com/docker/docker/issues/30402](https://github.com/docker/docker/issues/30402) and [forum.mailcow.email/t/solved-mailcow-docker-causes-kernel-panic-edit/448](https://forum.mailcow.email/t/solved-mailcow-docker-causes-kernel-panic-edit/448)
+
+  Fore more details read: [github.com/docker/docker/issues/30402](https://github.com/docker/docker/issues/30402) and [forum.mailcow.email/t/solved-mailcow-docker-causes-kernel-panic-edit/448](https://forum.mailcow.email/t/solved-mailcow-docker-causes-kernel-panic-edit/448)
 - Mailcow: dockerized requires [some ports](#default-ports) to be open for incomming connections, so make sure that your firewall is not bloking these. Also make sure that no other application is interferring with mailcow's configuration.
 - A correct DNS setup is crucial to every good mailserver setup, so please make sure you got at least the [basics](dns/#the-minimal-dns-configuration) covered bevore you begin!
 - Make sure that your system has a correct date and time setup. This is crucial for stuff like two factor TOTP authentication.
@@ -42,3 +43,35 @@ If you have a firewall already up and running please make sure that these ports
 | Dovecot POP3S       | TCP      | 995    | dovecot-mailcow |
 | Dovecot ManageSieve | TCP      | 4190   | dovecot-mailcow |
 | HTTP(S)             | TCP      | 80/443 | nginx-mailcow   |
+
+## Enabling NTP services
+
+To ensure that you have the correct date and time setup on your system, please check the output of `timedatectl status`:
+
+```bash
+$ timedatectl status
+      Local time: Sat 2017-05-06 02:12:33 CEST
+  Universal time: Sat 2017-05-06 00:12:33 UTC
+        RTC time: Sat 2017-05-06 00:12:32
+       Time zone: Europe/Berlin (CEST, +0200)
+     NTP enabled: yes
+NTP synchronized: yes
+ RTC in local TZ: no
+      DST active: yes
+ Last DST change: DST began at
+                  Sun 2017-03-26 01:59:59 CET
+                  Sun 2017-03-26 03:00:00 CEST
+ Next DST change: DST ends (the clock jumps one hour backwards) at
+                  Sun 2017-10-29 02:59:59 CEST
+                  Sun 2017-10-29 02:00:00 CET
+```
+
+The lines `NTP enabled: yes` and `NTP synchronized: yes` indicate wether you have NTP enabled and if it's syncronized.
+
+To enable NTP you need to run the command `timedatectl set-ntp true`. You also need to edit your `/etc/systemd/timesyncd.conf`:
+
+```
+# vim /etc/systemd/timesyncd.conf
+[Time]
+Servers=0.pool.ntp.org 1.pool.ntp.org 2.pool.ntp.org 3.pool.ntp.org
+```