You can't put a fixed value to time itself, because that is subjective. Fiat is used as a proxy for that.

Imagine this. A bus driver in Germany probably gets paid more than a bus driver in India. But at the end of the day they can roughly pay for the same amount of stuff for their work in their respective countries.

Now, of course there will be differences in how much you can get in both countries, because it's a complex system, with each country having different laws, incentives, etc. But at the end of the day, it is how people live and think about work.

Fiat is just a proxy to capture this idea, because time valuation changes over time. An inexperienced 18 year old would value their time less than an experienced worker in their 40s. Also, some people intrinsically value their time more than others.

That's why fiat (or bitcoin, or any kind of money) is used for exchanging goods and services instead of time.