Add description of the add() and sub() methods

Part of #2853

Author: veod32

doc/reference/reference_lua/datetime/datetime_object.rst

@@ -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. [TBD]
+
+        :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. [TBD]
+
+        :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>`. [TBD]
+
+        :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
+            ...