from datetime import datetime


def is_date_older(date1_str, date2_str):
    """
    Returns true if date1_str is older than date2_str

    :param date1_str: The first date
    :type date1_str: string
    :param date2_str: The second date
    :type date2_str: string
    :returns: boolean
    """
    first_date = datetime.strptime(date1_str, "%Y-%m-%dT%H:%M:%SZ")
    second_date = datetime.strptime(date2_str, "%Y-%m-%dT%H:%M:%SZ")
    return first_date <= second_date