Launching CalEventer will always use the current date as the Begin Date, with an initial default date range of 3 months. You can alter the range value in the Preferences Pane any time you wish, and the new value for the End Date will be immediately applied based on whatever date is in the Begin date button. Also, as you move the Begin Date back and forth, that interval will be maintained in the End Date field.
Understand that you are NOT locked into this End Date; it is merely there for your convenience. You can change the End Date at any time without affecting the Begin Date. However, any change in the Begin Date will be reflected in the End Date, so it is always best to set that first.