Time management requires self knowledge.
As a skill, time management is not about any specific system of structuring your time. It is about knowing how your energy and attention fluctuate and how to capitalize on that.
Attention is heavily influenced by your interest in the activity and the energy you have.
So we have at least 4 variables: time, attention, energy and interest. Three of them fluctuate and are affected by many things.
With experience and awareness we get to know our limits. That is when we start learning how to manage our time.