What do you mean by an “error” ? 
I mean, what happens, more precisely ?
And how are you trying to display the dates in the base ?
From a quick test in the sandbox vault, this:
---
dates:
date1: 2025-11-01
date2: 2025-11-30
---
… would return an object under the key dates (which Properties in Live Preview currently doesn’t really support)
If I create a base and display the dates key, I’ll get the same result: it’ll return the “raw” object.
So, to be able to use these values, I would need to dereference them where/when needed … in a formula property (for example) using something like:
date(dates.date2) // outputs 2025-11-30 (as a date object)
Is this the error you’re seeing ?