Kaitlyn, welcome to Tableau!
To elaborate a bit on Neil's solution:
You have a DATE column in your data set, and it contains either an actual date or nothing - which Tableau renders as NULL. There are a couple of ways you could deal with this.
One is a calculated field that turns a NULL into something else. In this case, you'd like it to say "TBD". That's a bit of a problem, though, because all the OTHER values in that column are dates, and TBD is a text string. That means that your new calculated field would have to be a TEXT field, not a date. You'd have to manually convert each real date into a text string.
That's a lot of work for a fairly simple problem, and that's why Neil's solution is better in this case. Just change what Tableau displays from NULL to whatever you want. That way the underlying dates stay as dates, and the empty slots get called something else entirely without messing up the data types.
If you get to the stage where there are OTHER potential values in the column, then you can think about a new calculated field to handle it, but for now, changing the alias is just fine!
Thank you, Neil and Michael, for your quick responses!
So, I'm not getting the option when I right click on the Null field to edit the alias. I'm wondering if it's because the main data type is listed as a date. I'm able to see the edit alias option on other fields, just not this one. Perhaps, there isn't too easy of a solution for this... I'm okay with leaving it as Null, if needed. I was just hoping there was a quick way to make the change to the label.
Yeah, you're right. Can't edit a date field. Shucks.
Calculated field it is, then, but that can be tricky with dates. Or, as you say, leave it alone.
It doesn't really make sense to "fix" the source data, right? Because there IS no start date yet for those rows.
Correct, currently, the data has TBD listed for individuals who've been hire, but have not selected a start date as of yet. I'm going to leave it as is for now.
Thank you, though! At least I've now learned how to edit aliases, even if it's not able to work in this scenario.