docs: add section for calendar events

and move the info defined in 'Schedules' there,
the explanation of calendar events is inspired by the systemd.time
manpage and the pve docs (especially the examples are mostly
copied/adapted from there)

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>

docs/administration-guide.rst

@@ -237,7 +237,7 @@ settings of how many backup snapshots for each interval of ``hourly``,
 ``daily``, ``weekly``, ``monthly``, ``yearly`` as well as a time-independent
 number of backups to keep in that store. :ref:`Pruning <pruning>` and
 :ref:`garbage collection <garbage-collection>` can also be configured to run
-periodically based on a configured :term:`schedule` per datastore.
+periodically based on a configured schedule (see :ref:`calendar-events`) per datastore.
 
 Creating a Datastore
 ^^^^^^^^^^^^^^^^^^^^
@@ -654,7 +654,7 @@ Sync Jobs
 
 Sync jobs are configured to pull the contents of a datastore on a **Remote** to a
 local datastore. You can either start a sync job manually on the GUI or
-provide it with a :term:`schedule` to run regularly. You can manage sync jobs
+provide it with a schedule (see :ref:`calendar-events`) to run regularly. You can manage sync jobs
 under **Configuration -> Sync Jobs** in the web interface, or using the
 ``proxmox-backup-manager sync-job`` command:
 

docs/calendarevents.rst

@@ -0,0 +1,100 @@
+
+.. _calendar-events:
+
+Calendar Events
+===============
+
+Introduction and Format
+-----------------------
+
+Certain tasks, for example pruning and garbage collection, need to be
+performed on a regular basis. Proxmox Backup Server uses a format inspired
+by the systemd Time and Date Specification (see `systemd.time manpage`_)
+called `calendar events` for its schedules.
+
+`Calendar events` are expressions to specify one or more points in time.
+They are mostly compatible with systemds calendar events.
+
+The general format is as follows:
+
+.. code-block:: console
+  :caption: Calendar event
+
+  [WEEKDAY] [[YEARS-]MONTHS-DAYS] [HOURS:MINUTES[:SECONDS]]
+
+Note that there either has to be at least a weekday, date or time part.
+If the weekday or date part is omitted, all (week)days are included.
+If the time part is omitted, the time 00:00:00 is implied.
+(e.g. '2020-01-01' refers to '2020-01-01 00:00:00')
+
+Weekdays are specified with the abbreviated english version:
+`mon, tue, wed, thu, fri, sat, sun`.
+
+Each field can contain multiple values in the following formats:
+
+* comma-separated: e.g.,  01,02,03
+* as a range: e.g., 01..10
+* as a repetition: e.g, 05/10 (means starting at 5 every 10)
+* and a combination of the above: e.g., 01,05..10,12/02
+* or a `*` for every possible value: e.g., \*:00
+
+There are some special values that have specific meaning:
+
+=================================  ==============================
+Value                              Syntax
+=================================  ==============================
+`minutely`                         `*-*-* *:*:00`
+`hourly`                           `*-*-* *:00:00`
+`daily`                            `*-*-* 00:00:00`
+`weekly`                           `mon *-*-* 00:00:00`
+`monthly`                          `*-*-01 00:00:00`
+`yearly` or `annualy`              `*-01-01 00:00:00`
+`quarterly`                        `*-01,04,07,10-01 00:00:00`
+`semiannually` or `semi-annually`  `*-01,07-01 00:00:00`
+=================================  ==============================
+
+
+Here is a table with some useful examples:
+
+======================== =============================  ===================================
+Example                  Alternative                    Explanation
+======================== =============================  ===================================
+`mon,tue,wed,thu,fri`    `mon..fri`                     Every working day at 00:00
+`sat,sun`                `sat..sun`                     Only on weekends at 00:00
+`mon,wed,fri`            --                             Monday, Wednesday, Friday at 00:00
+`12:05`                  --                             Every day at 12:05 PM
+`*:00/5`                 `0/1:0/5`                      Every five minutes
+`mon..wed *:30/10`       `mon,tue,wed *:30/10`          Monday, Tuesday, Wednesday 30, 40 and 50 minutes after every full hour
+`mon..fri 8..17,22:0/15` --                             Every working day every 15 minutes between 8 AM and 6 PM and between 10 PM and 11 PM
+`fri 12..13:5/20`        `fri 12,13:5/20`               Friday at 12:05, 12:25, 12:45, 13:05, 13:25 and 13:45
+`12,14,16,18,20,22:5`    `12/2:5`                       Every day starting at 12:05 until 22:05, every 2 hours
+`*:*`                    `0/1:0/1`                      Every minute (minimum interval)
+`*-05`                   --                             On the 5th day of every Month
+`Sat *-1..7 15:00`       --                             First Saturday each Month at 15:00
+`2015-10-21`             --                             21st October 2015 at 00:00
+======================== =============================  ===================================
+
+
+Differences to systemd
+----------------------
+
+Not all features of systemd calendar events are implemented:
+
+* no unix timestamps (e.g. `@12345`): instead use date and time to specify
+  a specific point in time
+* no timezone: all schedules use the set timezone on the server
+* no sub-second resolution
+* no reverse day syntax (e.g. 2020-03~01)
+* no repetition of ranges (e.g. 1..10/2)
+
+Notes on scheduling
+-------------------
+
+In `Proxmox Backup`_ scheduling for most tasks is done in the
+`proxmox-backup-proxy`. This daemon checks all job schedules
+if they are due every minute. This means that even if
+`calendar events` can contain seconds, it will only be checked
+once a minute.
+
+Also, all schedules will be checked against the timezone set
+in the `Proxmox Backup`_ server.

docs/epilog.rst

@@ -38,3 +38,6 @@
 .. _RFC3399: https://tools.ietf.org/html/rfc3339
 .. _UTC: https://en.wikipedia.org/wiki/Coordinated_Universal_Time
 .. _ISO Week date: https://en.wikipedia.org/wiki/ISO_week_date
+
+.. _systemd.time manpage: https://manpages.debian.org/buster/systemd/systemd.time.7.en.html
+

docs/glossary.rst

@@ -51,14 +51,3 @@ Glossary
       A remote Proxmox Backup Server installation and credentials for a user on it.
       You can pull datastores from a remote to a local datastore in order to
       have redundant backups.
-
-   Schedule
-
-      Certain tasks, for example pruning and garbage collection, need to be
-      performed on a regular basis. Proxmox Backup Server uses a subset of the
-      `systemd Time and Date Specification
-      <https://www.freedesktop.org/software/systemd/man/systemd.time.html#>`_.
-      The subset currently supports time of day specifications and weekdays, in
-      addition to the shorthand expressions 'minutely', 'hourly', 'daily'.
-      There is no support for specifying timezones, the tasks are run in the
-      timezone configured on the server.

docs/index.rst

@@ -37,6 +37,7 @@ in the section entitled "GNU Free Documentation License".
    command-syntax.rst
    file-formats.rst
    backup-protocol.rst
+   calendarevents.rst
    glossary.rst
    GFDL.rst