fix(extensions)!: correct return type for subtract:date_iday operation

Signed-off-by: Niels Pardon <par@zurich.ibm.com>

Author: nielspardon

extensions/functions_datetime.yaml

@@ -463,7 +463,7 @@ scalar_functions:
             value: date
           - name: y
             value: interval_day<P>
-        return: date
+        return: precision_timestamp<P>
   -
     name: "lte"
     description: less than or equal to

tests/cases/datetime/subtract_datetime.test

@@ -2,14 +2,14 @@
 ### SUBSTRAIT_INCLUDE: '/extensions/functions_datetime.yaml'
 
 # timestamps: examples using the timestamp type
-subtract('2016-12-31T13:30:15'::ts, 'P5D'::iday) = '2016-12-26T13:30:15'::ts
+subtract('2016-12-31T13:30:15'::ts, 'P5D'::iday<0>) = '2016-12-26T13:30:15'::ts
 subtract('2016-12-01T13:30:15'::ts, 'P5Y'::iyear) = '2011-12-01T13:30:15'::ts
-subtract('2016-12-01T13:30:15'::ts, 'PT5H'::iday) = '2016-12-01T08:30:15'::ts
+subtract('2016-12-01T13:30:15'::ts, 'PT5H'::iday<0>) = '2016-12-01T08:30:15'::ts
 
 # date: examples using the date type
-subtract('2020-12-31'::date, 'P5D'::iday) = '2020-12-26'::date
+subtract('2020-12-31'::date, 'P5D'::iday<0>) = '2020-12-26T00:00:00'::pts?<0>
 subtract('2020-12-31'::date, 'P5Y'::iyear) = '2015-12-31'::date
 subtract('2020-12-31'::date, 'P5M'::iyear) = '2020-07-31'::date
 
 # null_input: examples with null args or return
-subtract(null::date?, 'P5D'::iday) = null::date?
+subtract(null::date?, 'P5D'::iday<0>) = null::pts?<0>