Updating description with timeline

CesiumJS
1

I have a large number of data samples of objects moving around and I want to add location and description samples to each object (entity) so that I can use the timeline to see them smoothly move forward/backward.

Most importantly, I need the description to update to whatever it was at the time when you move the timeline back to.

I have my front-end receiving the json data fine, and I can see how to put the locations in from the Interpolation example using SampledPositionProperty.

I am stuck on two things:
1. How do I have the entitys' descriptions change when you drag the timeline back/forwards?
2. How do I constrain the timeline's range to the first/last data points, but have its stop constraint move with the clock? So, you should always be able to drag the timeline up to the current time, but no further.

2

In case anyone sees this later and wants to know how to do this, you can use:

var description_composite = new Cesium.TimeIntervalCollectionProperty();

Use that as the desctiption field for the entity.

And then add descriptions to it like this:

description_composite.intervals.addInterval(new Cesium.TimeInterval({
                    start : time,
                    stop : next_time,
                    isStartIncluded : true,
                    isStopIncluded : true,
                    data : description_string
                }));