from datetime import datetime


def to_isoformat(date_str: str) -> str:
    """
    Last-modified to isoformat
    Sat, 27 Jun 2020 02:00:18 GMT => 2020-06-27T02:00:18
    """
    return datetime.strptime(date_str, "%a, %d %b %Y %H:%M:%S %Z").isoformat()