Inconsistency: Generate timestamp for proof with `T` separator in ISO 8601 format

[cognivore]

Why?

JS doesn't care much about having T in ISO 8601:

But we would rather be consistent, especially since some other parsers may not be so kind to us when it comes to T-less ISO 8601s.

What?

• [ ] Firgure out what causes the missing T
• [ ] Change that code in such way that there is no T missing.