Activity Saved Search > Ability to get the number of days a task is opened.

This can be done by adding a Formula(Numeric) that uses formula string shown below.

REGEXP_SUBSTR({completeddate}-{date}, '[^ ]+')

Here are the steps:
I. Create an activity saved search.
1. Navigate to Lists > Search > Saved Searches > New.
2. Click Activity.
3. In the Criteria tab, set Status= is any of Completed, In Progress, Not Started
4. In the Results tab, add Formula(Numeric) | REGEXP_SUBSTR({completeddate}-{date}, '[^ ]+')
5. Click Save & Run.

This will populate the number of days a task is still opened.