code: public DateTime minusMillis(int millis) {
          if (millis == 0) {
              return this;
          }
          long instant = getChronology().millis().subtract(getMillis(), millis);
          return withMillis(instant);
      }
docs: Returns a copy of this datetime minus the specified number of millis.
  <p>
  The calculation will subtract a duration equivalent to the number of
  milliseconds.
  <p>
  The following three lines are identical in effect:
  <pre>
  DateTime subtracted = dt.minusMillis(6);
  DateTime subtracted = dt.minus(Period.millis(6));
  DateTime subtracted = dt.withFieldAdded(DurationFieldType.millis(), -6);
  </pre>
  <p>
  This datetime instance is immutable and unaffected by this method call.
  
  @param millis  the amount of millis to subtract, may be negative
  @return the new datetime minus the increased millis
  @since 1.1