Document methods and logic of interval arithmetics in the datetime module

doc/reference/reference_lua/datetime.rst

@@ -28,19 +28,35 @@ Below is a list of the ``datetime`` module functions and methods.
         *   -   :ref:`format() <datetime-format>`
             -   Convert the standard presentation of a ``datetime`` object into a formatted string.
 
-        *   -   :ref:`totable() <datetime-totable>`
+        *   -   :ref:`datetime_object:totable() <datetime-totable>`
             -   Convert the information from a ``datetime`` object into the table format.
 
         *   -   :ref:`set() <datetime-set>`
             -   Update the field values in the existing ``datetime`` object.
 
+        *   -   :ref:`parse() <datetime-parse>`
+            -   Convert an input string with the date and time information into a ``datetime`` object.
+
+        *   -   :ref:`add() <datetime-add>`
+            -   Modify an existing datetime object by adding values of the input arguments.
+
+        *   -   :ref:`sub() <datetime-sub>`
+            -   Modify an existing datetime object by subtracting values of the input arguments.
+
         *   -   :doc:`./datetime/interval_new`
             -   Create an ``interval`` object from a table of time units.
 
+        *   -   :ref:`interval_object:totable() <interval-totable>`
+            -   Convert the information from an ``interval`` object into the table format.
+
+        *   -   :doc:`./datetime/interval_arithm`
+            -   Arithmetic operations with datetime and interval objects.
 
 ..  toctree::
     :hidden:
 
     datetime/new
     datetime/datetime_object
     datetime/interval_new
+    datetime/interval_object
+    datetime/interval_arithm

doc/reference/reference_lua/datetime/datetime_object.rst

@@ -11,7 +11,7 @@ datetime_object
 
     ..  method:: totable()
 
-        Convert the information from the ``datetime`` object into the table format.
+        Convert the information from a ``datetime`` object into the table format.
         Resulting table has the following fields:
 
         ..  container:: table
@@ -92,14 +92,14 @@ datetime_object
 
     ..  _datetime-format:
 
-    ..  method:: format( ['convensions'] )
+    ..  method:: format( ['input_string'] )
 
         Convert the standard ``datetime`` object presentation into a formatted string.
-        The formatting convension specifications are the same as in the `strftime <https://www.freebsd.org/cgi/man.cgi?query=strftime&sektion=3>`__ library.
-        Additional convension for nanoseconds is `%f` which also allows a modifier to control the output precision of fractional part: `%5f` (see the example below).
-        If no arguments are set for the method, the default convensions are used: `'%FT%T.%f%z'` (see the example below).
+        The conversion specifications are the same as in the `strftime <https://www.freebsd.org/cgi/man.cgi?query=strftime&sektion=3>`__ library.
+        Additional specification for nanoseconds is `%f` which also allows a modifier to control the output precision of fractional part: `%5f` (see the example below).
+        If no arguments are set for the method, the default conversions are used: `'%FT%T.%f%z'` (see the example below).
 
-        :param string convensions: string consisting of zero or more conversion specifications and ordinary characters
+        :param string input_string: string consisting of zero or more conversion specifications and ordinary characters
 
         :return: string with the formatted date and time information
         :rtype: string
@@ -243,3 +243,140 @@ datetime_object
             - 1970-01-01T03:00:00.125+0300
             ...
 
+    ..  _datetime-add:
+
+    ..  method:: add( input[, { adjust } ] )
+
+        Modify an existing datetime object by adding values of the input argument.
+
+        :param table input: an :ref:`interval object <interval-new>` or an equivalent table (see **Example #1**)
+        :param string adjust: defines how to round days in a month after an arithmetic operation.
+                                Possible values: ``none``, ``last``, ``excess`` (see **Example #2**). Defaults to ``none``.
+
+        :return: datetime_object
+        :rtype: cdata
+
+        **Example #1:**
+
+        ..  code-block:: tarantoolsession
+
+            tarantool> dt = datetime.new {
+                        day = 26,
+                        month = 8,
+                        year = 2021,
+                        tzoffset  = 180
+                        }
+            ---
+            ...
+
+            tarantool> iv = datetime.interval.new {day = 7}
+            ---
+            ...
+
+            tarantool> dt, iv
+            ---
+            - 2021-08-26T00:00:00+0300
+            - +7 days
+            ...
+
+            tarantool> dt:add(iv)
+            ---
+            - 2021-09-02T00:00:00+0300
+            ...
+
+            tarantool> dt:add{ day = 7 }
+            ---
+            - 2021-09-09T00:00:00+0300
+            ...
+
+        ..  _datetime-add-example2:
+
+        **Example #2:**
+
+        ..  code-block:: tarantoolsession
+
+            tarantool> dt = datetime.new {
+                        day = 29,
+                        month = 2,
+                        year = 2020
+                        }
+            ---
+            ...
+
+            tarantool> dt:add{month = 1, adjust = 'none'}
+            ---
+            - 2020-03-29T00:00:00Z
+            ...
+
+            tarantool> dt = datetime.new {
+                        day = 29,
+                        month = 2,
+                        year = 2020
+                        }
+            ---
+            ...
+
+            tarantool> dt:add{month = 1, adjust = 'last'}
+            ---
+            - 2020-03-31T00:00:00Z
+            ...
+
+            tarantool> dt = datetime.new {
+                        day = 31,
+                        month = 1,
+                        year = 2020
+                        }
+            ---
+            ...
+
+            tarantool> dt:add{month = 1, adjust = 'exсess'}
+            ---
+            - 2020-03-02T00:00:00Z
+            ...
+
+    ..  _datetime-sub:
+
+    ..  method:: sub( { input[, adjust ] } )
+
+        Modify an existing datetime object by subtracting values of the input argument.
+
+        :param table input: an :ref:`interval object <interval-new>` or an equivalent table (see **Example**)
+        :param string adjust: defines how to round days in a month after an arithmetic operation.
+                                Possible values: ``none``, ``last``, ``excess``. Defaults to ``none``.
+                                The logic is similar to the one of the ``:add()`` method -- see :ref:`Example #2 <datetime-add-example2>`.
+
+        :return: datetime_object
+        :rtype: cdata
+
+        **Example:**
+
+        ..  code-block:: tarantoolsession
+
+            tarantool> dt = datetime.new {
+                        day = 26,
+                        month = 8,
+                        year = 2021,
+                        tzoffset  = 180
+                        }
+            ---
+            ...
+
+            tarantool> iv = datetime.interval.new {day = 5}
+            ---
+            ...
+
+            tarantool> dt, iv
+            ---
+            - 2021-08-26T00:00:00+0300
+            - +5 days
+            ...
+
+            tarantool> dt:sub(iv)
+            ---
+            - 2021-08-21T00:00:00+0300
+            ...
+
+            tarantool> dt:sub{ day = 1 }
+            ---
+            - 2021-08-20T00:00:00+0300
+            ...

doc/reference/reference_lua/datetime/interval_arithm.rst

@@ -0,0 +1,95 @@
+.. _interval_arithm:
+
+Interval arithmetic
+===================
+
+Since :doc:`2.10.0 </release/2.10.0>`.
+
+The :doc:`datetime module </reference/reference_lua/datetime>` enables creating objects of two types: ``datetime`` and ``interval``.
+
+If you need to shift the ``datetime`` object values, you can use either the modifier methods, that is, the :ref:`:add <datetime-add>` or :ref:`:sub <datetime-sub>` methods,
+or apply interval arithmetic using overloaded `+ (__add)` or `- (__sub)` methods.
+
+``:add``/``:sub`` modify the current object, but ``+``/``-`` create copy of the object as the operation result.
+
+In the interval operation, each of the interval subcomponents are sequentially calculated starting from the largest (``year``) to the smallest (``nsec``):
+
+*   ``year`` -- years
+*   ``month`` -- months
+*   ``week`` -- weeks
+*   ``day`` -- days
+*   ``hour`` -- hours
+*   ``min`` -- minutes
+*   ``sec`` -- seconds
+*   ``nsec`` -- nanoseconds.
+
+If results of the operation exceed the allowed range for any of the components, an exception is raised.
+
+The ``datetime`` and ``interval`` objects can participate in arithmetic operations:
+
+*   The sum of two intervals is an interval object, which fields are sum of each particular component of operands.
+
+*   The result of subtraction of two intervals is similar: it's an interval object where each subcomponent is the result of subtraction of particular fields in the original operands.
+
+*   If you add datetime and interval objects, the result is a datetime object. Addition is being performed in a determined order from the largest component (``year``) to the smallest (``nsec``).
+
+*   Subtraction of two datetime objects produce an interval object. Difference of two time values is performed not as difference of the epoch seconds,
+    but as difference of all the subcomponents, that is, years, months, days, hours, minutes, and seconds.
+
+*   An untyped table object can be used in each context where the typed datetime or interval objects are used if left operand is typed object with overloaded operation of ``+`` or ``-``.
+
+The matrix of the ``addition`` operands eligibility and their result types:
+
+..  container:: table
+
+    ..  list-table::
+        :widths: 25 25 25 25
+        :header-rows: 1
+
+        *   -
+            -   datetime
+            -   interval
+            -   table
+
+        *   -   **datetime**
+            -
+            -   datetime
+            -   datetime
+
+        *   -   **interval**
+            -   datetime
+            -   interval
+            -   interval
+
+        *   -   **table**
+            -
+            -
+            -
+
+The matrix of the ``subtraction`` operands eligibility and their result types:
+
+..  container:: table
+
+    ..  list-table::
+        :widths: 25 25 25 25
+        :header-rows: 1
+
+        *   -
+            -   datetime
+            -   interval
+            -   table
+
+        *   -   **datetime**
+            -   interval
+            -   datetime
+            -   datetime
+
+        *   -   **interval**
+            -
+            -   interval
+            -   interval
+
+        *   -   **table**
+            -
+            -
+            -

doc/reference/reference_lua/datetime/interval_new.rst

@@ -3,22 +3,22 @@
 datetime.interval.new()
 =======================
 
-..  function:: datetime.interval.new( [{ units }] )
+..  function:: datetime.interval.new( [{ input }] )
 
     Since :doc:`2.10.0 </release/2.10.0>`.
 
     Create an object of the :ref:`interval type <index-box_interval>` from a table of time units.
     See :ref:`description of units <interval-new-args>` and :ref:`examples <interval-new-example>` below.
 
-    :param table units: Table of :ref:`time units <interval-new-args>`. For all possible time units, the values are not restricted.
+    :param table input: Table with :ref:`time units and parameters<interval-new-args>`. For all possible time units, the values are not restricted.
                                 If an empty table or no arguments are passed, the ``interval`` object with the default value ``0 seconds`` is created.
 
-    :return: :doc: interval object
+    :return: interval_object
     :rtype: cdata
 
     ..  _interval-new-args:
 
-    **Possible input time units for ``datetime.interval.new()**
+    **Possible input time units and parameters for ``datetime.interval.new()**
 
     ..  container:: table
 
@@ -72,6 +72,11 @@ datetime.interval.new()
                 -   number
                 -   0
 
+            *   -   adjust
+                -   Defines how to round days in a month after an arithmetic operation.
+                -   string
+                -   'none'
+
     ..  _interval-new-example:
 
     **Examples:**