code: function(t_start) {
  		if (t_start) {
  			BOOMR.addVar("rt.tstart", t_start);
  		}
  		if(typeof impl.t_start === "number" && impl.t_start !== t_start) {
  			BOOMR.addVar("rt.cstart", impl.t_start);
  		}
  		BOOMR.addVar("rt.bstart", BOOMR.t_start);
  		if (BOOMR.t_lstart) {
  			BOOMR.addVar("rt.blstart", BOOMR.t_lstart);
  		}
  		BOOMR.addVar("rt.end", impl.timers.t_done.end);	// don't just use t_done because dev may have called endTimer before we did
  	}
docs: Writes a bunch of timestamps onto the beacon that help in request tracing on the server
  - rt.tstart: The value of t_start that we determined was appropriate
  - rt.cstart: The value of t_start from the cookie if different from rt.tstart
  - rt.bstart: The timestamp when boomerang started
  - rt.end:    The timestamp when the t_done timer ended
  
  @param t_start The value of t_start that we plan to use