Formatting date using Summarize Posts widget

I’m using the Summarize Posts widget to display upcoming shows in the side bar, https://www.unionbrewery.ca.

To sort the shows by date I need to store the date in MySQL date format yyyy-mm-dd.

How could I modify the display format of the field in the Summarize Posts widget?

I’m using the Define Search tool of Summarize Posts. Is there way to do that manually so I have more control?

Thanks for any tips!