Merge pull request #447 from DrDoof/feat/ntp

ntp: keep the time from an NTP server

doc/ntp.md

@@ -0,0 +1,33 @@
+# Time
+
+The switch has no battery backed clock. It counts seconds from power on, and
+it can take the date and time from an NTP server once the network is up. The
+time is lost at every restart and set again at the first synchronisation.
+
+```
+ntp server <name|ip>     # default pool.ntp.org
+ntp interval <1-1440>    # minutes between two synchronisations, default 60
+ntp timezone <+hh:mm>    # offset from UTC, -12:00 to +14:00
+ntp dst off|eu|us        # daylight saving rule
+ntp on
+ntp off
+ntp                      # settings and time
+time                     # local date and time
+```
+
+The client speaks SNTP version 4 and synchronises at once when turned on,
+then after every interval. A server given by name is resolved through the DNS
+resolver before every synchronisation. An answer from an unsynchronised
+server is ignored, and a failed attempt is repeated after 30 seconds.
+
+The local time is UTC plus the time zone offset, plus one hour while daylight
+saving time applies:
+
+* `eu`: from 01:00 UTC on the last Sunday of March to 01:00 UTC on the last
+  Sunday of October;
+* `us`: from 02:00 local time on the second Sunday of March to 02:00 local
+  time on the first Sunday of November.
+
+The same settings, with the current local time and the last synchronisation,
+are in the Time card of the System page. Settings made there or on the
+console are kept only once the configuration is saved.