Possible to set 2 data fields within 1 select option?
You could use the Preparse plugin to create a field that is automatically updated with the eventDate value whenever the entry is saved. It wouldn't be a datetime field, but if you stored the timestamp (i.e. {{ dateString|date('U') }}) it'd be easy to use as such.

Another option is to have an hidden input in your form for the eventDate datetime field, and use JavaScript to set its value whenever the &lt;select&gt; input changes (i.e. listen for the change event for the latter, in your JS).

The third option would be to write a plugin which listens to the entries.beforeSaveEntry event in order to set the eventDate value from the dropdown field.