As a matter of interest, a vessel voyage has two starts and two finishes. F.a.o.p. to E.o.p. and Berth to Berth.
Since often, a ship will be delayed at the pilot station waiting on a berth it was usual to use Pilot to pilot, i.e. FullAway on Passage to End of Passage.
The distance from Belfast to Southhampton as per the route tables in force in 1912 is shown as 492 nautical miles.- pilot to pilot.
At the end of the voyage, the logbook would record what was known as G.a.s. - General average speed. This was found by dividing the total distance steamed by the time between the moments of F.a.o.p. and E.o.p recorded in the bridge Movement Book.
I have no doubt this was how Lightolier got his average speed of 18 knots, which suggests that the run time was 492 divided by 18 = 27 hours 20 minutes. Thus, if F.a.o.p. was, say, 20-00hrs on April 2nd, then Titanic slowed down for the Nabb pilot at 23-20 pm Belfast time - 23-45 GMT on the 3rd
A vessel does not arrive at a port until she crosses the port limits, that is when she starts to accrue port dues. In the case of Southampton, Titanic would have needed to steam up the East Solet until she crossed the port limits - another 12 miles after the Pilot boarded. This works well with the 1-15 am entry into the Harbour Master's arrivals book.