Subtract 2 fields of a data table in Ruby on Rails

I have the following problem.

I have a table lets call it Activity.

In this table there are 3 fields one is the date_start, the second is date_end and the third is the activity_type, which correspond the date that the activity start and ends and the type of activity for example physical activity and eating.

I would like to find the median time period that a activity_type lasts.

For example:

Activity.where(activity_type: "Eating healthy")

So I want to apply in this question, a sum statement with the time difference between the date_start and date_end.

How I can find the time differences of all the same type activities?