TimeRange contains two properties Start and End which defines an extent of time. Either property can be set to null. When Start is null it means the time range begins infinite in the past and when End is null this means it ends infinite in the future. This is useful for example when setting the MapView.Time property to show all time before a given date, all time after a given date or to just show all time.

Reference

TimeRange Members
ArcGIS.Desktop.Mapping Namespace