How to validate that date in future in reference to another date?

You're gonna have to bear down and write yourself a Validator.

This should get you started:

Cross field validation with Hibernate Validator (JSR 303)

You should not use Annotations for cross field validation, write a validating function instead. Explained in this Answer to the Question, Cross field validation with Hibernate Validator (JSR 303).

For example write a validator function like this:

public class IncomingData {

  @FutureOrPresent
  private Instant startTime;

  @Future
  private Instant endTime;

  public Boolean validate() {
      return startTime.isBefore(endTime);
  }
}

Then simply call the validator function when first receiving the data:

if (Boolean.FALSE.equals(incomingData.validate())) {
  response = ResponseEntity.status(422).body(UNPROCESSABLE);
}