gml:timeInterval conforms to ISO 11404 which is based on floating point values
for temporal length. ISO 11404 syntax specifies the use of a positiveInteger together with
appropriate values for radix and factor. The resolution of the time interval is to one radix
^(-factor) of the specified time unit. The value of the unit is either selected from the
units for time intervals from ISO 31-1:1992, or is another suitable unit. The encoding is
defined for GML in gml:TimeUnitType. The second component of this union type provides a
method for indicating time units other than the six standard units given in the
enumeration.