def post_reply_time(post, conv):
    """
    Returns the time between the post and its parent

    Parameters
    ----------
    post : UniMessage
        The message

    conv : Conversation
        A collection of posts

    Returns
    -------
    float
       The time between the `post` and its parent. If multiple parents, returns the minimum response difference
    """
    if post.created_at is None:
        return -1

    diffs = [
        abs((post.created_at - conv.posts[rid].created_at).total_seconds())
        for rid in post.reply_to if rid in conv.posts and conv.posts[rid].created_at is not None
    ]

    if not diffs:
        return -1

    return min(diffs)