The `expireAfter` method, accepting duration and timeUnit parameters, has been added to the JwtBuilder interface. This method calculates the JWT expiration date as either the issue time plus the duration or the system current time plus the duration if an issuedAt time hasn't been set. Additional tests for this feature have been included in `DefaultJwtParserTest.groovy`.

Two new tests have been added to DefaultJwtParserTest to validate JWT expiration behavior. The tests ensure that for the 'expireAfter()' method, duration must be more than 0 and timeUnit cannot be null. The error messages for these validation checks have also been modified for clarity.

…h_takes_a_duration

The token expiry calculation logic has been cleaned up and optimized in DefaultJwtBuilder. A previously used optional stream has been replaced with a more straightforward if-else structure, leading to ease of code maintenance and improved readability.

…_which_takes_a_duration' into 60-convenience_expiration_setter_which_takes_a_duration

A typographical error in the comments of the 'exp' function in JwtBuilder was corrected. The phrase "specified it if" was changed to "specified if it", making the comments clearer and easier to understand.

…ration_setter_which_takes_a_duration