You do not need (or have) to work it out. You get this from each payload in the stream, so when you parse each payload, keep saving / replacing the timepoint in the (latest) payload to a variable. That way, when you need to reconnect (as you do when the stream resets after 24 hours), you specify the (optional) timepoint that you saved in the variable. This timepoint helps ensure that you do not miss any updates that might have streamed past while you were disconnected.
NOTE: timepoints are only effective for a set period of time (possibly 10 days … maybe a bit more).