fn to_s(_: &mut VM, self_val: Value, args: &Args2) -> VMResult {
    args.check_args_num(0)?;
    let time = self_val.as_time();
    let s = match time {
        TimeInfo::Local(t) => format!("{}", t.format("%Y-%m-%d %H:%M:%S %z")),
        TimeInfo::UTC(t) => format!("{}", t.format("%Y-%m-%d %H:%M:%S UTC")),
    };
    Ok(Value::string(s))
}
Time#to_s -> String
https://docs.ruby-lang.org/ja/latest/method/Time/i/to_s.html
ruruby/src/builtin/time.rs