There's a few ways you could do it, from resetting at intervals, ie, each clock hour or in relation to the start time, or you could reset it with a click at anytime of your choosing.
Or you could setting it as a rolling time, ie, its always looking at the new hi/lo in the last 5min, 30mins 1hr etc
Most are already done in an example somewhere so once you know how you want to define the time frame I should be able to point you at some files