In source are string values instead of dates?

I'm not sure without testing it out, does MS Query use worksheet or VBA syntax in formulas, so you try either
...>= TEXT([enter start date], "yyyymmdd") AND ... <= TEXT([enter end date], "yyyymmdd")
or
...>= FORMAT([enter start date], "yyyymmdd") AND ... <= FORMAT([enter end date], "yyyymmdd")