How do I set the duration of the output of a job in Elastic Transcoder using Django?

I want to automate using Amazon's Elastic Transcoder to trim a video file in Django. My code so far is:

def trim_video(key, new_key, duration):

    pipeline_id = 'XXXXXXXXXXXX-XXXXXX'
    region = 'XXXXXXXX'

    transcoder_client = boto.elastictranscoder.connect_to_region(region)

            'pipeline_id': pipeline_id,
            'input_name': {'Key': key},
            'output': {
                    'Key': new_key,
                    "PresetId": 'XXXXXXXXXXXXX-XXXXXX'
    print('Job has been created. The output key will be ' + new_key)

This code will cause the file to be transcoded but will not trim it. What do I add in order to trim the video?