Hi, you shouldn't need to store timestamps in your events.... when the events are triggered there is already a system level timestamp associated to them...
This plugin should automatically do the math for you between event15 and event16...
There are several examples in the documentation that show some of the options, including if the timer should reset if your first event is triggered again (without getting to the second event, and other potential events that can trigger a reset, or customizing the cookie to use to maintain the timer between sessions, etc).
Since your implementation is fairly simple... start and end help chat, you should only need the simplest version of the code:
s.eVar46 = getTimeBetweenEvents("event15", true, "event16", true);
I've not actually used his plugin myself... I am just going by the documentation... the fact that the format of the data could potentially change if not specified (as per above), you could make it a bit more absolute:
s.eVar46 = getTimeBetweenEvents("event15", true, "event16", true, "s_tbe", 0, "s");
This should force it to always be counted in seconds, and reset at the end of the session.