There's not gonna be a proper answer that applies to everyone. For myself, riding BMX flatland, riding unicycles, carving wood, learning survival skills, keeping time..
Check my username. I've been partly obsessed with keeping accurate track of time since I learned to read an analog clock at age 9.
By age 12, I started learning the exact times of the school bells. By age 15, I learned how to rebuild digital watches, and even replace the quartz crystal with a more accurate one.
By age 17 I was rebuilding mechanical self-winding wristwatches, and also learning to overclock computers.
Edit: For extra clarity, I also now know how to tune the firing order on an ICE engine, no matter how many cylinders. I also know how to time a VCR and tune a guitar.
Sounds like you should pursue a career at NIST so your hobby can align with a profession. They're all about keeping track of time to extreme precision with atomic and optical clocks.
I saved your comment to respond later once I got my words together.
I really appreciate your comment, seriously. But I never thought of it as a hobby, I thought of it as an obligation, to understand time, as best as possible anyways.
At age 9, I had just recently gotten my first glasses. I was left home alone for like a half hour, and I just stared at their analog clock. After 5 minutes, counting the ticks and watching the dials, I just understood it. Never even had to ask an adult.
I always thought of it as an obligation of education that I somehow missed before I got glasses.
I never once thought of it as a hobby before you described it that way.
Also, a 1 meter pendulum swings at a rate of once per second. Handy info to know if all the clocks shut down, like in a survivalist situation or natural disaster.