Converting Date to OffsetTime can rely on the OffsetTime.toInstant() method as follows:

// e.g., 08:03:20.336+02:00
public static OffsetTime dateToOffsetTime(Date date) {

  return OffsetTime.ofInstant(dateToInstant(date), DEFAULT_TIME_ZONE);
}

Converting OffsetTime to Date should take into account that OffsetTime doesn't have a date component. This means that the solution should set the date at January 1, 1970, the epoch:

// e.g., Thu Jan 01 08:03:20 EET 1970
public static Date offsetTimeToDate(OffsetTime offsetTime) {

  return Date.from(offsetTime.atDate(LocalDate.EPOCH).toInstant());
}