You will probably have to do this in two steps: sort by location and date, get the average of the location/date grouping, then use the subquery approach in another query where this first query is the source.
that is closest to the average
I think you'll also have to decide if the closest number is to be greater than or lesser than the average and use either choice in the criteria, or else introduce another step. Maybe a query that has two calculated fields to get the Min value that is greater than the Avg and the Max value that is less than the Avg. Those values would be the criteria in the last query.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.