Just for a bit of background information:
In Kirby 2, a date field called
date was a special field. This difference between a date field called
date and a date field called whatever, doesn’t exist anymore. To turn a date field into a date in Kirby 3, you use the
toDate() method on any date field, either without a date format to get a Unix timestamp, or with a format to get a formatted date.
The first example you posted above
Site()->children()->find(‘invoices’)->children()->listed()->filterBy(‘date’, ‘>’, ‘2020-05-6’)
->filterBy(‘date’, ‘<’, ‘2019-04-5’);
should therefore also work with any date field no matter its name, as long as you use the same date format that is stored, so with two digits for the day, I guess.