Hi,

There is no XML file for this; the Day/Week/Month/Quarter/Year groupby options on date fields in pivot views are generated entirely in JavaScript, not via XML views.

In modern Odoo the relevant file is:

addons/web/static/src/views/pivot/pivot_model.js

And for the dropdown rendering:

addons/web/static/src/search/group_by_menu/group_by_menu_generator.js

The available date granularities are defined as a static list in JS. To add "Hour", you'd need to extend that list in a custom module and also ensure _read_group_process_groupby on the Python side handles the hour granularity; which in Odoo 8 it likely doesn't natively, so you'd need to patch both sides.

For Odoo 8 specifically, the JS file to look at is:

web/static/src/js/views/pivot_view.js

Search for "week" or "quarter" in that file; the granularity array will be right there.