import calendar


def _iso_8601_to_timestamp(utc):
    """Given a UTC 8601 time string in the form YYYY-MM-DDTHHMMSSZ,
    return the associated Unix timestamp.

    Arguments:
        utc (str): a UTC 8601 formatted string in the form YYYY-MM-DDTHHMMSSZ
                   where T and Z are constants and the remaining letters
                   are substituted by the associated datetime elements.
    """
    # Read the datetime elements from the string
    year = int(utc[0:4])
    month = int(utc[5:7])
    day = int(utc[8:10])
    hour = int(utc[11:13])
    mins = int(utc[13:15])
    secs = int(utc[15:17])
    # Put the elements in the from of a time struct
    t = (year, month, day, hour, mins, secs, -1, -1, 0)
    # Use calendar to convert the time struct into a Unix timestamp
    return calendar.timegm(t)