DOCS-14215 add dateDiff and update week start

Author: mungitoperrito

source/includes/extracts-agg-operators.yaml

@@ -379,6 +379,10 @@ content: |
       * - Name
         - Description
 
+      * - :expression:`$dateDiff`
+
+        - Returns the difference between two dates.
+
       * - :expression:`$dateFromParts`
 
         - Constructs a BSON Date object given the date's constituent

source/includes/fact-timezone-description-no-option.rst

@@ -0,0 +1,29 @@
+The timezone of the operation result. ``<tzExpression>`` must be a 
+valid :ref:`expression <aggregation-expressions>` that resolves to a
+string formatted as either an `Olson Timezone Identifier
+<https://en.wikipedia.org/wiki/List_of_tz_database_time_zones>`_ or a
+`UTC Offset <https://en.wikipedia.org/wiki/List_of_UTC_time_offsets>`_.
+If no ``timezone`` is provided, the result is displayed in ``UTC``.
+
+.. list-table::
+   :header-rows: 1
+   :widths: 30 70
+
+   * - ``Format``
+     - ``Examples``
+
+   * - `Olson Timezone Identifier`
+
+     - ::
+
+         "America/New_York"
+         "Europe/London"
+         "GMT"
+
+   * - `UTC Offset`
+
+     - ::
+
+         +/-[hh]:[mm], e.g. "+04:45"
+         +/-[hh][mm], e.g. "-0530"
+         +/-[hh], e.g. "+03"

source/reference/operator/aggregation.txt

@@ -312,6 +312,10 @@ Alphabetical Listing of Expression Operators
      - Returns the hyperbolic cosine of a value that is measured in
        radians.
 
+   * - :expression:`$dateDiff`
+
+     - Returns the difference between two dates.
+ 
    * - :expression:`$dateFromParts`
 
      - Constructs a BSON Date object given the date's constituent
@@ -1007,6 +1011,7 @@ Alphabetical Listing of Expression Operators
    /reference/operator/aggregation/convert
    /reference/operator/aggregation/cos
    /reference/operator/aggregation/cosh
+   /reference/operator/aggregation/dateDiff   
    /reference/operator/aggregation/dateFromParts
    /reference/operator/aggregation/dateToParts
    /reference/operator/aggregation/dateFromString