code: public final boolean isValidField(final int field)
  	{
  		if (field < 0 || field >= FIELD_MASKS.length)
  			throw new KNXIllegalArgumentException("illegal field");
  		return !isBitSet(0, FIELD_MASKS[field]);
  	}
docs: Returns whether a field is valid, i.e., supported and used for date/time
  information.
  <p>
  Only data of valid fields are required to be set or examined. A field set to not
  valid shall be ignored, it is not supported and contains no valid data.<br>
  Possible fields allowed to be set valid or not valid are {@link #YEAR},
  {@link #DATE}, {@link #TIME}, {@link #DAY_OF_WEEK} and {@link #WORKDAY}.
  
  @param field field number
  @return <code>true</code> if field is supported and in use, <code>false</code>
  otherwise