def SUBTRACT(x, y):
    """
    Subtracts two numbers to return the difference,
    or two dates to return the difference in milliseconds,
    or a date and a number in milliseconds to return the resulting date.
    See https://docs.mongodb.com/manual/reference/operator/aggregation/subtract/
    for more details
    :param x: The number or field of number
    :param y: The number or field of number (is subtracted from the first argument)
    :return: Aggregation operator
    """
    return {'$subtract': [x, y]}