I have used dialog plugin and previously used Troi. Is there a plugin that allows dropdown calendars in a custom dialog?

Off the top of my head I don't know about the plugin route.

 

With the new window options in 12, and especially if you are on mac, you can just use a new window with a layout that simulates a dialog.  Then have away with the Dropdown calendar field options.

Yes - but my idea was to not create a new record unless criteria was met. Wouldn't I have to create a new record within the layout in order to populate the date field? I was hoping to use global fields within a dialog, do the check, then create a new record if it passes.

Just use a separate table with global fields.  Do the same test.  The only difference, instead of calling the plugin or custom dialog, you Go to Layout [ ] instead.