What is fixed time productivity?
We all agree that a day is 24 hours long. We sleep for between 6 to 8 hours every day. Leaving us with 16 hours to get stuff done. If we all have the same amount of time, how are some people getting way more done? Do they have more time?
No.
Part of the solution is admitting to ourselves that we have limited time. Accepting these limits are the basis of "fixed time" productivity.