Personally, I believe date calculation is one of the most useful calculations in Tableau. However, it could be confusing with all the functions if you are not familiar with Tableau calculation language. Therefore, I created this cheat sheet for you (and myself) to refer to when formulating date calcs.
It is important to know your data type before putting it into a calculation. Hence, this cheat sheet is created based on data types. In order to use this, you first need to find out what data type you have, it should be either string, integer or date. It is represented by the small boxes on top. After that, follow the arrows to achieve your desired outputs. I hope you find that helpful. Let me know if you think there is any improvement that can be made on the cheat sheet.