To do this, the date fields would have to match in the data source. Therefore, the best recommendation would be to remove the timestamp from the table that has them.
I guess you could also use custom SQL to drop the time part as you bring both in
There are many options available, the route to use depends on what analysis you want to perform. You may want to try appending the data sources with a union, or you can create a third data source that has every possible date time value and use that as the primary.