The custom fields of type Daterange and Date allow you to either set a date range or a single date.
You can create a custom field of type Number by going to the Fields tab in the workspace settings.
Once you click on the + Add field button, a window with all the available field types will appear.
Choose Date range to define a period of time or Date for a single date.
When your field is ready, hit the Add field button and it will appear in the list of all your custom fields on the left. Keep in mind that you can also change the order of the fields using drag and drop.
IMPORTANT: The order of this list affects the column order on your table view and the order of your custom fields in the item panel.
SOON: This will also determine the order of your custom fields on your item cards.
Here is an example of Date and Date range in the Table view: