Hi @avalon.ds ,
I use M code to get the first and last date on my sales or fact table and then add an amount of days to it:
Normal date table:
M date table:
The difference is in the Source, you create a normal date table then go to the source and use M language.
List.Min(FactTable[DateColumn]) => this will give you the start date of your FactTable data
List.Sum({List.Max(FactTable[DateColumn]), #duration(30, 0, 0, 0)}) => this will get the last date on your FactTable and #duration will add the amount of days you decide.
Also, be careful in always adding {} inside the List.Sum, otherwise you will get an error.