Return true if the cache file holding list of all datasets
    does not exist or is older than 30 days

def check_cache(path):
    """Return true if the cache file holding list of all datasets
    does not exist or is older than 30 days
    """
    if not os.path.exists(path):
        return True
    else:
        # check the age
        mod_date = datetime.fromtimestamp(os.path.getmtime(path))
        if mod_date < (datetime.now() - timedelta(days=30)):
            return True
        else:
            return False