Whenever I say this people have questions and doubts, so let's go over the process.
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A1. You get better at prioritizing: important stuff gets done. Less important stuff is—not important.
A2. You get better at planning. You don't have the luxury of wasting time messing around.
codewithoutrules.com/2018/02/11/wor…
A. Negotiate at your current job. Replacing employees is expensive: you know the code, business, who to talk to, culture, etc.. If you're willing to walk away from your job if need be, you can often get what you ask for.
A. Apply to job, don't tell them you want <40 hours a week. Because you're now more effective and more productive, you'll be desirable employee. When you get offer, time to start negotiating for hours. And you have proof that this is workable.
A. Most will say "no". Some will say yes. Here's a programmer who has been working <40 hours a week, as employee, for 15 years: codewithoutrules.com/2018/01/08/par…
A. You might be interested in my book, The Programmer's Guide to a Sane Workweek: codewithoutrules.com/saneworkweek/