How Can I add Custom Data/Time filed variable in description or multi-text custom field?

[Shah Baloch]

There is a change control project, and we added default values such as a table in a custom multi-text field. Whenever a user tries to create an issue, the file already has a table with filed values. That part is working fine. There are two custom date start/end fields, and we want to include those two field variables in the default value. Is it possible to add a custom date/time variable either in a multi-text custom file or description? An example screenshot is attached. 

Appreciate your help!