Ask HN: What's your way to tracking time spend?

I'm imaging a way to tracking time: Using some NFC stickers and NFC feature of a smart phone. Each sticker stand for different event or task. And when I am dong "task-A", I could place the phone on "Sticker-A", meanwhile a app of my phone start to record time for "task-A". When I change another sticker, timer run into another recording process.

What's your recommend way to tracking time?