Tableau really works best at visualizing data. Creating data or extra data cannot be added in Tableau Desktop. The data would need to be added to the underlying data set via the data source editor(wheter it be excel or updating a data base table)
There is a way to show or hide empty columns from the data set in Tableau.
Please correct my understanding if I did not propely answer your question.
You might try blending your actual data to a new data source that has your targets for all dates.
There are limitations... but, if you don't need to do a lot of filtering, this would work.
Post data if you need more help.
Can you post your data ir fake data and explain what is needed
All, thanks but I solved it this morning with a date formula after joining the 2 tables:
IF ISNULL([Data source 1]) THEN [Date source 2]
ELSE [Data source 1] END