code: public DateTime toDateTime(ReadableInstant baseInstant) {
          Chronology chrono = DateTimeUtils.getInstantChronology(baseInstant);
          long instantMillis = DateTimeUtils.getInstantMillis(baseInstant);
          long resolved = chrono.set(this, instantMillis);
          return new DateTime(resolved, chrono);
      }
docs: Resolves this partial against another complete instant to create a new
  full instant. The combination is performed using the chronology of the
  specified instant.
  <p>
  For example, if this partial represents a time, then the result of this
  method will be the datetime from the specified base instant plus the
  time from this partial.
  
  @param baseInstant  the instant that provides the missing fields, null means now
  @return the combined datetime