Good morning,
If you want to do this via Formula Queries, then you can convert the time of day into decimal hours. You can then use a Sum Values on that field and then divide by the count to get the average. If you need it to be in a TimeOfDay format, then you can convert it back into TimeOfDay.
If you need to use a Formula Query because you need for it to be valid for many records in a different table, then I would suggest creating a 1 record table and connect it to the records in the Table you are trying to Average as a Parent, and then you can use the Average Summary Field Function, and then you can use a Formula Query to retrieve that single value. If you need it in the Child Table of that then you can just use a Lookup field.
Good Luck!
John
