After installing JWT, head over to Administration → Issues

Browse to Custom fields

Create your first custom field

from the Custom fields admin section, head over to Add custom Field → Advanced then add the desired field offered by JWT:

• JWT calculated date-time fields
• JWT calculated number fields
• JWT calculated text fields

On the configuration screen of your newly created custom field go to Actions → Configure

Start by adding an expression that is used to calculate the field. Screenshot

Optionally: Customize the display format. Screenshot

To learn more about configuring the display formats, check out the following page.