Howdy Tommy,
That is not yet a feature of our calendar. But, we do have an official feature request (click here) that you could vote your support for.
There are some complexities with implementing this. Where would a TBA event show up on Month, Week, and Day views if it has no date? Where would it show up on the list views? My favorite idea that has been suggested thus far is for you to select a tentative date around the time you think it will be, and show it there. But, make it clear with bold owrding that there is no specific date yet, and the date/time are still TBA. If this solution would work for you and you have good theming experience, it might be possible already to build this by add a custom field that sets an event to TBA, and then modifying all of the front end templates to not show a date if that field is true for an event. This is pretty complicated to build though given all the templates you will need to modify.
To summarize, it’s not a feature yet but please vote on the feature request. If you’re an enterprising and experienced PHP dev, you might be able to build it using the idea I shared above. Does that answer your question?
Cheers!
– Brook