https://stackoverflow.com/questions/39108549/switchmapto-creates-observable-before-subscription
It is calling early because you are calling the function when you are building the stream.
switchMap takes a function which gets invoked when a new value comes from up stream. The result of that function is subscribed to as a new stream.
switchMapTo takes an Observable which is subscribed to for every value that comes from up stream. You are creating the Observable by invoking getTestStartTime$().
Use switchMap when the output depends on the value coming from upstream, and use switchMapTo when the actual upstream value is irrelevant and is only used as a signal for subscription.
