Currently, the first date or date/time field listed in a report type layout is automatically used in the time frame filter, but it is not possible to control the default value set for the time frame filter. It would be great to have control over this setting within the configuration of the custom report type.
The default value is generally "Current FQ", but it seems to vary for some standard report types.
This could be implemented on the Edit Layout screen of the custom report type editor within the properties for date and date/time fields. Bonus, this could allow for greater control over which field is used instead of simply using the first field present in the layout.