The article "Notice in writing ahead of 30 days" is nothing but a procedure labours have to carry out and not a term the one-sided unconditioned warning right of cancellation must meet.

“提前30日书面通知”仅是劳动者主观履行的程序，而非其单方预告解除权客观所需满足的条件。

youdao

One is unconditioned.

一个是无条件作用。

耶鲁公开课 - 心理学导论课程节选