To return a future or past date in Tableau use DATEADD
DATEADD(date_part, interval, start_date)
The date_part can be:
Add 1 month to 2017-09-01
DATEADD('month', 1, #2017-09-01#)
Find the date 7 days ago
DATEADD('day', -7, TODAY())
These calculations come in handy when you need to look at project deadlines, forecasts or look back at advertising spend from 7 days ago to calculate ROI.