Queries :: Select Record With Nearest Datetime?

I would like to add fields from one table to another based on matching datetime codes.

Most data are in the table 'metadata' (information on sound recordings), and i want to add lat long fields from a large table of GPS points 'gpsData'. Each record in the metadata table has start and end datetime fields - i want to a add lat and long that matches the start time and a lat and long that matches the end time (so four fields to metadata).

My approach has been to try this with two separate select queries (i.e. one for adding the start locations and one for adding the end locations), with the intention of then combining these two outputs in a single table.

When i try a simple select query though I can see that:

a) there are no matching time codes for some records, because although the hh:mm:ss look the same there are differences in the lesser decimal places of the datetime serial numbers. I have tried using the round function on the datetime fields of both tables but this doesn't fix it (I don't understand why not).

b) there are some duplicates because there are sometimes more than one GPS point for each second, and the metadata times are given to second precision (so some metadata records get two entries in the query result).

How can I have the query select the gpsData record with the nearest datetime stamp to that in the metadata table's start time field? The respective field names are UTC and StartUTC. Can i do the same for the end times simultaneously or should i do this separately?