Better time display behaviour.

Specifically:

(1) if the inter-sample interval is six hours or less, the last-sample
timestamp omits the date.

(2) If the inter-sample interval is six hours or less, and the
estimated finish date is the same as the (non-displayed) timestamp
date, the finish time omits the date.

(3) The last-sample timestamp is displayed even in the "only one sample
so far" state.