The standard specifies that RANGE-type frame bounds support date[time] arithmetics. That is, one can write queries like this:

SUM(number) OVER (PARTITION BY col1 

                  ORDER BY date_col

                  RANGE BETWEEN INTERVAL 1 MONTH PRECEDING 

                  AND INTERVAL 1 MONTH FOLLOWING)

MariaDB only supports numeric types there. The reason for the limitation is that interval is not a first-class datatype in MySQL codebase.

• Interval is not parsed to an Item object
• One cannot use addition/subtraction to add an interval to a date.
• 
  MDEV-9676 uses Item_func_plus/Item_func_minus, which allow range bounds to use any numeric datatype. This doesn't include datetime, though.

The building blocks for date+interval arithmetics are already there:

SELECT DATE_ADD(date_col, INTERVAL 10 DAY) 

SELECT DATE_SUB(date_col, INTERVAL 5 YEAR) 

That is,

• the parser is able to parse interval values.
• there is code to do addition/subtraction.

This task is to support interval arithmetics in window functions.