Cesium entity availability forever

i have multiple model which some of them should be available just in time between their start and stop time in their porisionProperty . but some of them should be available for ever . how can i tell the second part be available for ever? i set the stoptime simething far like 2223.but they disappear from the first moment and never show up.this is my code:

> const availability= new Cesium.TimeIntervalCollection([ new Cesium.TimeInterval({ start: entity.startdate, stop: 8296172396 }) ]);