In addition to Text, Number and Drop Down Custom Fields, now you can add a Date custom field. To create one, go to Settings > Custom Fields. Select the part of the app where you want to add the custom field (Deals, Projects, Invoices...) and click on Add Custom Field.
From the menu, select Date field:
Set a name and a description for the custom field. In this example, a custom field named "Expected Closure Date" was created.
Hit Create Field and go to the section of Productive where you've set it to be. Be sure to add the custom field from the Column menu.
Clicking on the column of the custom field will open a date picker:
Select the date you'd like to set:
The date is now added:
You can use this custom field as a filter, group by it, and you can use it in Insights.