[IMP] web: accurate relative dates and rename RemainingDaysField

Luxon is not very accurate when the field is close to today: If today is
Apr 30, so a deadline set to May 1 will be displayed as "Next month".
In practice, it is not wrong, but it is not very accurate.

The widget name 'RemainingDaysField' was misleading as it implies
a focus on future countdowns, whereas the widget is also used for past
dates.

task-6175442

Author: jucop-odoo

content/applications/studio/fields.rst

@@ -190,7 +190,7 @@ Date (`date`)
 
 The :guilabel:`Date` field is used to select a date on a calendar.
 
-- :guilabel:`Remaining Days`: the remaining number of days before the selected date is displayed
+- :guilabel:`Relative Date`: the remaining number of days before the selected date is displayed
   (e.g., *In 5 days*), based on the current date. This field should be set to :guilabel:`Read only`.
 
 .. example::
@@ -255,12 +255,13 @@ To add a date range:
    .. image:: fields/date-time-examples.png
       :alt: Examples of Date & Time fields with different widgets
 
-Remaining Days (`remaining_days`)
-*********************************
+Relative Date (`relative_date`)
+*******************************
 
-The :guilabel:`Remaining Days` widget displays the remaining number of days before the selected date
-(e.g., *In 5 days*), based on the current date and time. This field should be set to :guilabel:`Read
-only`.
+The :guilabel:`Relative Date` widget displays displays a human-readable relative calendar string
+computed from the difference between the field value and today (e.g. *"yesterday"*, *"today"*,
+*"in 3 days"*, *"next month"*). The exact format depends on the magnitude of the difference.
+This field should be set to :guilabel:`Readonly`.
 
 .. _studio/fields/simple-fields-checkbox:
 

content/developer/reference/frontend/javascript_reference.rst

@@ -808,10 +808,18 @@ Date Range (`daterange`)
 
             <field name="start_date" widget="daterange" options="{'end_date_field': 'end_date'}" />
 
-Remaining Days (`remaining_days`)
+Relative Date (`relative_date`)
     This widget can be used on date and datetime fields. In readonly, it displays
-    the delta (in days) between the value of the field and today. The widget turns
-    into a regular date or datetime field in edit mode.
+    a human-readable relative calendar string computed from the difference between
+    the field value and today (e.g. *"yesterday"*, *"today"*, *"in 3 days"*,
+    *"next month"*). The exact format depends on the magnitude of the difference:
+
+    - Within ±30 days: the unit is forced to *days* (e.g. *"in 5 days"*, *"3 days ago"*).
+    - Between 31 and 99 days: Luxon picks the most appropriate unit automatically
+      (e.g. *"next month"*, *"2 months ago"*).
+    - Beyond ±99 days: the widget falls back to the formatted date string.
+
+    In edit mode the widget falls back to a regular date/datetime field.
 
     - Supported field types: `date`, `datetime`