So in the example supplied the view is at the level of Day and yet there are many orders per day. I am curious if the goal in this example is actually to find the difference in date/time at the level of Order? Understanding the level of detail is important to figure out the average... I like to start with a crosstab view of all of the data and calculate a value in Tableau first before going to a chart view.
A view of the sample data with an expected result - would help a lot to assist you here.