Note that each task has two different IDs:
id = The GUID of the task, store this and use it when updating the task.
shortId = A shorter more memorable version of the task ID which you can show to the user, you can also use it to generate a link to the task:
The planned finish date must be less than one year after the planned start date. If you require a later date than this you will need to create the task first, then perform a Task Update to set the extended finish date.