This type of read-only custom field has a value which is dynamically calculated from a custom math/numeric expression.
It can be used to obtain a numeric value depending on the value of other fields in the current issue or in any other issues (linked issues, sub-tasks, epics, stories, JQL selected issues, etc).
Configuring a calculated date-time field is straightforward. You have to configure a valid parser expression, which will "calculate" the output, and optionally a custom display format.
Create a new Calculated Date-Time Field (by JWT) custom field and name it properly.
Providing a description will help you to identify what the custom field does but this step is optional.
Add a valid parser expression which will "calculate" the output.
Usually field codes will be used for this purpose.
The result of the calculation must be a valid number.
Optionally choose a custom display format format. This is extremely useful to enhance usability for end users.
Read more about the available Display formats.