calendar module: set of possible event-durations to small

- in calendar module
- try to add an event

result:
- only several event-durations can be created

expected:
- event-duration can be set arbitrarily

comment:
- arbitrary duration values should include day-exceeding events

- e.g. a team member can be on sick leave for 2 weeks , a part group of the project can be on conference for 3 days, ...