HRB is the local time on board ship. None of my reference books actually use the term Nautical Time and all examples of logs and non-astronomical navigation calculations use "time" for local time. Any other time is explicitly named, for example Universal Time.
As distance increases from the Meridian, time changes about 1 hour every 15 degrees. Therefore, the time on board ship is adjusted as the vessel moves from one time zone to another. Before GPS longitude was calculated by comparing Greenich Mean Time on a dedicated chronometer to local time read at noon, that is when the sun is at its zenith.