Given a list of tags, returns the datetime of the tag with the given name; Otherwise None.

def dateof(tag_name, tags):
    """Given a list of tags, returns the datetime of the tag with the given name; Otherwise None."""
    for tag in tags:
        if tag['name'] == tag_name:
            commit = read_url(tag['commit']['url'])
            return parse_timestamp(commit['commit']['committer']['date'])
    return None