Select Edit \ System Options \ Dates. There is a box for Peak Date date and time ranges.
Alternatively, you can right-click inside graphs by date and time to set the peak date and time ranges.
While looking at a graph by date, place your cursor inside the graph at your 'from' date, right-click and Set <date> to > Peak 'From' Date.
Do the same for the 'to' date...
While looking at a graph by time, place your cursor inside the graph at the 'from' time, right-click and Set <time> to > Peak 'From' Time.
Do the same for the 'to' time...
These date and time ranges apply to all peak day graphs found under Graphs > Peak Day.
The time range is not indicated in the title of the graph, so you might want to add a footnote to the graph if you intend to share it.
The Peak Date and Time ranges are remembered, so if you forget to reset them, be aware. You have to remember to reset these ranges. This can be done with the check box in the Edit > System Options > Dates window, or with a right-click inside a graph.
The Peak Date / Time range is also used for all the 'Top XYZ' graphs. For example, Graphs > Current Day > Jobs > Top Jobs/CPU and Graphs > Jobs/Subsystems > Top Jobs > CPU use the date and/or time ranges. In these 'top' graphs, the time range is indicated in the title of the graphs.