DOCSP-11788: [Charts] As an author, I can filter dates to a current time period (#407)

Author: steveren

source/filter-chart-results.txt

@@ -139,14 +139,17 @@ options. You can filter fields with the following data types:
         relative to the time the chart is rendered (e.g., the last six
         months).
 
+      - A date range based on the current time when the chart is rendered,
+        with a choice of periods.
+
       - An absolute date range, which is a range
         between specific begin and end dates.
 
       Select the appropriate tab for more information on the date filter:
 
       .. tabs::
 
-         .. tab:: Relative Date Filter
+         .. tab:: Relative
             :tabid: relative-date
 
             Relative date filters specify a range relative to the 
@@ -181,7 +184,69 @@ options. You can filter fields with the following data types:
                   :scale: 70%
                   :alt: Image showing relative date filter
 
-         .. tab:: Absolute Date Filter
+         .. tab:: Period
+            :tabid: period-date
+
+            Period date filters specify a range relative to the current
+            time when the chart is rendered. To define an period date range,
+            select :guilabel:`Period` at the top of your date filter card.
+
+            Period options include:
+        
+            - Hour (from the top of the hour)
+            - Day (from midnight)
+            - Week (from midnight Sunday)
+            - Month (from midnight on the 1st of the month)
+            - Year (from midnight on the 1st of January)
+    
+            The Period date filter has two usage options:
+    
+            - **Current**. The start date is the beginning of the selected
+              period before the current time, and the end date is the
+              end of the selected period. For example, the :guilabel:`Month`
+              option includes only data from the current month, starting at
+              midnight on the first day of the month and extending until
+              the last day of the month.
+    
+            - **Previous**. The start date is the beginning of the previous
+              period, and the end date is the start of the most recent
+              period before the current time. For example, the :guilabel:`Day`
+              option includes only data from the day before the current day
+              up until midnight of the current day.
+
+            .. list-table::
+               :header-rows: 1
+
+               * - To use the **Current** option:
+                 - To use the **Previous** option:
+
+               * - 1. Select :guilabel:`Current` from the left dropdown
+                      menu.
+
+                   #. Choose a time period from the right dropdown menu.
+                      Chart data is restricted to the time period from
+                      the beginning of the selected period to the end of
+                      the selected period.
+
+                 - 1. Select :guilabel:`Previous` from the left dropdown
+                      menu.
+
+                   #. Choose a time period from the right dropdown menu.
+                      Chart data is restricted to the time period from
+                      the beginning of the selected period to the beginning
+                      of the next period after the selected one.
+
+            .. example::
+
+               The following period date filter only shows documents
+               with a ``released`` field that is in the current week,
+               starting from midnight of the most recent Sunday.
+
+               .. figure:: /images/charts/period-date.png
+                  :scale: 70%
+                  :alt: Image showing period date filter
+
+         .. tab:: Absolute
             :tabid: absolute-date
 
             Absolute date filters use absolute dates to define their upper and lower