This calculated field will return the creation date of the linked epic.
Configuration
Create a new Calculated Date-Time Field (by JWT) custom field and name it appropriately.
Providing a description will help you to identify what the custom field does, but this step is optional.
Parser expression
Add the following expression:
first(fieldValue({issue.created}, linkedIssues("has Epic"))) #since the function fieldValue() returns a list the function first() returns the single value of the list.
{issue.created} is the field code for the numeric value of Date and time of creation.
To achieve this, the following functions have been used: