No need to create a tyrannical government to force people to do deeds of arbitrary value for elderly people.
A money market account is insured and the real return is 0%, so it's not as vulnerable as you seem to think.
You can call money a credit. Then you can assign credits to actions and create a moral police to assign credits to people who do these actions and enter those credits into a leger. And now you've recreated communist China. Congrats.
All around, money is a better currency of exchange than time, or any other measure really. It measures well the actual value of time , effort, what have you. 1-to-1 time currency would be particularly bad choice, just imagine how much time you would have to pay back for someone who was born 60 years ago. Life was much much slower and unproductive. In the same amount of time today you can do multiple times the work/product that they did.