Replace `chrono::DateTime` with `chrono::Duration`

[tmplt]

From rust-embedded/itm#41, it's concluded that a ns-accurate baseline timestamp is hard to achieve which means that we'll seldom report an accurate timestamp, even if we have accurate offsets from target reset time.

To simplify the API, we should only report the duration since target reset as the timestamp. If the end-user wants an accurate timestamp, they can create it on their own by handling the baseline summartion on their own, which is trivial.