By default only sickness event types are allowed to recorded in the past but you can change this behavior by going to the “Administration |Event Types and Allowances" and then editing the event type you wish to modify. You will see the page as below. You have to check/un-check the “Enable In The Past” check box to modify this option.
When the enable in the past option is selected, Employees and Managers can request that event type to be in the past. Administrators can request any event type to be in the past whether the option is checked or not.