def calculate_delta_date(literal):
    """Calculate the date in the past from the given literal
    :param literal: A date literal, e.g. "today"
    :type literal: str
    :returns: Date between the literal and today
    :rtype: DateTime
    """
    mapping = {
        "today": 0,
        "yesterday": 1,
        "this-week": 7,
        "this-month": 30,
        "this-year": 365,
    }
    today = DateTime(DateTime().Date())  # current date without the time
    return today - mapping.get(literal, 0)