def when(string, timezone='UTC', prefer_dates_from='current_period'):
    """"Returns a MayaDT instance for the human moment specified.

    Powered by dateparser. Useful for scraping websites.

    Examples:
        'next week', 'now', 'tomorrow', '300 years ago', 'August 14, 2015'

    Keyword Arguments:
        string -- string to be parsed
        timezone -- timezone referenced from (default: 'UTC')
        prefer_dates_from -- what dates are prefered when `string` is ambigous.
                             options are 'past', 'future', and 'current_period'
                             (default: 'current_period'). see: [1]

    Reference:
        [1] dateparser.readthedocs.io/en/latest/usage.html#handling-incomplete-dates
    """
    settings = {
        'TIMEZONE': timezone,
        'RETURN_AS_TIMEZONE_AWARE': True,
        'TO_TIMEZONE': 'UTC',
        'PREFER_DATES_FROM': prefer_dates_from,
    }

    dt = dateparser.parse(string, settings=settings)
    if dt is None:
        raise ValueError('invalid datetime input specified.')

    return MayaDT.from_datetime(dt)