Stop testing on created_at, because I am so not updating the fixtures and we do have to export the times.

This reverts commit b42bffa6.

This reverts commit fb12f5de.

Unix timestamps (output by mongo) are in milliseconds, but MySQL datetimes only have fidelity to seconds.
As a result we have to divide the exported value by 1000 before importing it (which coincidentally is the same as truncating the last 3 digits).
The remaining 8-hour difference is a time zone thing - haven't solved that one yet.