Welcome to the forum - greet to have you here!
This is an excellent question, and even experienced Power BI users may have very different answers and strategies in response to your question. Here’s how I would handle it:
I would absolutely start with Enterprise DNA Expert @Melissa’s incredible Extended Date Table. This will have the vast majority of fields you will ever need, including one called ISAFTERTODAY which can be used to easily separate historical data from future forecasts.
Melissa and I did a whole series of videos on time intelligence uses the extended date table that you can find on the portal site…In particular focus on the initial video, the one on offsets and the one on ISAFTERTODAY.
Generally, calculated columns are the least preferred choice for adding additional capabilities to date table – if there are columns you need regularly that are not included, best to modify the M code that creates the date table to add those (we can help you with that if needed). Otherwise, just build the conditions into the different measures you will use in your analysis.
I hope this is helpful.
– Brian