Transform to DateTime

**Problem**

When rendering a timestamp generated with `bigint-time` to the user, the nanoseconds don't really matter, but I somehow need to turn it into a Time instance for formatting.

**Improvement**
Add a `toDate` method that allows to get the data as a standard JavaScript Date Object.

**Alternatives**
Maybe a `toISODate` method may be even better to use?

Start by locating the package entry point and the existing timestamp conversion logic. Implement the requested conversion to a standard JavaScript Date object, then verify that the returned value represents the timestamp correctly; the issue does not name a test file or specific entry point.

Written by the indexing model from the issue text.