Browse to Custom fields.

After installing JWT, head over to Administration → Issues

Create your first custom field

from Custom fields 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

To add an expression click on Edit Math/Time Expression and click on Edit Display format to configure the display format

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