web/web_datetime_picker_default_time/readme/USAGE.md

Static Default Time You can define the default time as follows for a static value For widget="datetime":

    <field name="your_datetime_field" widget="datetime" options="{'defaultTime': {'hour': 8, 'minute': 30, 'second': 15 }}"/>

For widget="daterange":

    <field name="your_start_datetime_field" widget="datetime" options="{'end_date_field': 'your_end_datetime_field', 'defaultStartTime': {'hour': 2, 'minute': 22, 'second': 22,}, 'defaultEndTime': {'hour': 3, 'minute': 33, 'second': 33,}}"/>

Dynamic Default Time Otherwise you can also use a JSON field to make it dynamic through a compute function, and reference this field in the view:

   start_time = field.Json(compute="_compute_start_time")

   def _compute_start_time(self):
       for rec in self:
           rec.start_time = {'hour': 8, 'minute': 30, 'second': 15 }

   <field name="start_time" invisible="1" />
   <field name="your_datetime_field" options="{'defaultTime': 'start_time'}"/>