Google Maps API provides TimeZone service via Map API. So TimeZone is auto selected in the TimeZone dropdown list based on location changed by the user. 

OpenStreetMap API don’t provides TimeZone service via Map API.

With OSM Map active you can still use Google’s TimeZone API service if you have set API Key in GeoDirectory > Settings > General > Google Maps API Key. Get the Google API Key with TimeZone Service enabled.

In Add Listing form by default the TimeZone field will automatically be set to the default location. After the listing editor changes the location(latitude & longitude) GD will automatically request an updated timezone from the Google Maps API for the new location. On a successful response it selects the updated TimeZone from the list of options.