How can I subtract a date from a fixed date?

This is to ask anyone to show me on how to calculate dates in ODK. I want to develop a new variable giving a that subtracts a date from a fixed date, how can you keep the calculation in ODK?

Not quite sure exactly what you want here - 'distance' (ie difference) between 2 dates, or subtracting (days? months? ...) from one date to get another. But you probably want to look at using decimal-date-time() to convert your dates to # days (from epoch) before performing any calculations on them. And date() will do the inverse; convert # days back to a more user friendly date.