I do get that error when trying to create a time_between via API.
If it is created via the UI first, I do not get the error when updating the property via API, so if something more complex is needed — like including a min, max, or math — updating a UI created time between via API can get you there.
If your just trying to create a whole bunch of properties via API (like moving things from a sandbox to production), this error is very problematic.
Based on the error text, I even gave adding this a try, to no effect.
"numberDisplayHint": "duration"
It seems they have not exposed it to the API, so you currently have to create via the UI to have that get set properly. I would guess this is what is being used behind the scenes for the display of number properties in general (unformatted, formatted with thousands separators, formatted as duration etc.), because that has never been exposed to the API, and we have not been able to set it for calculation_equation properties — it has just defaulted to formatted with thousands separators. [*update edit* There is now a number format selection in the UI when creating a calculation property there, so they are likely actively implementing this way of selecting a number format.]
@ShamashA You should create a new thread for this, if you haven’t already.