It’s important that you choose the right kind of planner for your unique style. Alarm clocks and timers are extremely valuable tools. If you are not good at estimating time, start timing your tasks and keep a record of how long things actually take.
Part-time, photo by Eke Miedaner. From the comments: "Shy clock?" "The clock is two-faced, so you have to take a look on both sides to know the time." (This is an official school-clock in KV Baden-Zurzach).