def get_start_date(participants, host_dict, repo_dates, repo):
    """ Finds start date based on participants.

    Checks if host participants have start dates associated with them.

    Args:
        participants: users in the pull request.
        host_dict: dictionary with host logins as keys and start dates as
                   values.
        repo_dates: dictionary mapping repositories to start dates.
        repo: current repository.

    Returns:
        Start date or "unknown".
    """
    # Special case in which host account has been deleted
    if repo not in repo_dates:
        for user in participants:
            try:
                cur_username = user["login"]
            except (TypeError, KeyError):
                continue
            if cur_username in host_dict:
                start_date = host_dict[cur_username]
                repo_dates[repo] = start_date
                return start_date
        return "unknown"
    return repo_dates[repo]