Get Event Elapsed Time
By using Get Event Start Time and subtracting from it the current timestamp, we can find for how long an event has been running:
var event = LViS.getEvent();
var startAt = event.getStartAt();
// Use the server time when working with
// other timestamps returned by the API
var timestamp = LViS.Date.now();
var eventElapsedTime = startAt - timestamp;
Additionally, as Get Event Start Time returns the value including the Sync delay, the code above would provide you the time elapsed since the start of the event plus the sync delay you've specified.
As an example, if you are building an on-demand streaming service feature in your app, and set the sync delay to match when the user first starts watching the video, the calculation above should match how long the user has been watching.