Example:

const constantProperty = ...;
const sampledProperty = ...;

//Create a composite property from two previously defined properties
//where the property is valid on August 1st, 2012 and uses a constant
//property for the first half of the day and a sampled property for the
//remaining half.
const composite = new Cesium.CompositeProperty();
composite.intervals.addInterval(Cesium.TimeInterval.fromIso8601({
    iso8601 : '2012-08-01T00:00:00.00Z/2012-08-01T12:00:00.00Z',
    data : constantProperty
}));
composite.intervals.addInterval(Cesium.TimeInterval.fromIso8601({
    iso8601 : '2012-08-01T12:00:00.00Z/2012-08-02T00:00:00.00Z',
    isStartIncluded : false,
    isStopIncluded : false,
    data : sampledProperty
}));

See:

• CompositeMaterialProperty
• CompositePositionProperty