Money merely is a place holder for time.
Some folks time is more valuable that others.
I don’t think money is a placeholder for time. Money is a placeholder for work. It allows me to separate my work from my reward by a period of time, but money is not that time.
Now, I might look at how much money I make for an hour of work, and that would translate to “time is money”. I’d know that I could make X dollars an hour doing my job. Then I’d look at everything else I could do for an hour, and say “is that worth X dollars”/
I used to live like that, and it made it very hard to take vacation, especially when my employer would pay me for excess vacation at the end of the year. But as I got older, my time became more valuable to me, so it was easier to take off of work.
On the other hand, I’ve done odd side jobs that paid me almost nothing per hour; I’d be driving to such a job, thinking that the entire day of work would be about one hour of my real job. In that case though, I was deciding that the work was enjoyable enough that it was worth the money difference.
The reason it is so important to have a stable currency is that you need people to believe that money is a good medium for exchanging the value of work. I don’t want to have to find something I can do for the guy who fixes my car, so that we can exchange equivalent work time. I want to do my own job, get money, take it to the repair shop, and have them repair my car. Money makes an efficient economy, but not when that money changes value wildly from day to day or week to week.
And if we are talking money, going UP in value is as problematic for the economy as going DOWN in value. You don’t want money to be an investment.