Custom range - To search by a custom date range, eg. a fortnight, click in the 'custom range' box and select a start and end date from the drop-down calendar, then click 'apply'.
Specific range - To view a specific time period, eg. a week or month, use the 'view' select buttons on the right.
To navigate forward and back to a specific time period selected, use the arrow keys. These will ignore any custom time period entered.
Appointment calendar view - The calendar view header panel is a little different. You can select the number of days in the week to show. Custom date ranges can't be used on the calendar view and this box is disabled.