def get_tm_json_url(cam_name=None, ds=None):
    """
    Get the url that contains the information about the frame captured times

    Input:
        cam_name: camera name
        ds: date string, e.g., 2019-01-03
    """
    return "https://tiles.cmucreatelab.org/ecam/timemachines/%s/%s.timemachine/tm.json" % (cam_name, ds)