Simplify DatePair.hasStartDate and DatePair.hasEndDate
implement Delete
fix bug of test case for delete
change wrong return type for Delete()
Fix logic bug in DatePair.isWithinPeriod which happens when two dates are the same
Add tests for null dates in DatePair.isWithinPeriod
implement updateTask() in excecute()
change update test case
delete the TODO comment in deleteTest
resolve markTaskCompleted TODO
change update() return comment