NavList:

Backward resources (which unfortunately includes many British and British-derived sources for beginners) use zone time when setting up celestial navigation scenarios and exercises. This is just plain stupid. I wish to welcome all of you to the 21st century in which UT (or "GMT", if we must...) is always available and easily maintained and displayed on a cheap spare watch. All celestial navigation work, from sight-taking through analysis and position plotting, should be done in UT. Never, never, never use zone time. But be alert to the backward practice...

You also asked if the time in question might be longitude converted to time. Another option would have been time based on the Sun (the "hour angle" of the Sun can be calculated from its altitude so if the hour angle is 45° 00.0', and the Sun is climbing, then we can say that the time is exactly 9:00:00). These are both viable options, and both were used a century ago and earlier.

Of the two options here, the first longitude-varying time is called "Local Mean Time". It's time based on Greenwich, but varying continuously with longitude. It can also be realized by longitude as time (my longitude is 4h48m west of Greenwich, so my LMT is always UT/GMT - 4h48m). This time was popular in the early decades of mechanical clocks when each locality could use one clock as a standard, but it was superseded by zone time.

The second longitude-varying time is called "Local Apparent Time" (or equivalently "Sundial Time", and it is, in fact, exactly what is read by a common sundial). Navigators 150 years ago commonly set their watches to this. In fact, this time by the Sun was known as "true time" especially before the mid-19th century and in some contexts (not usually in English) was called "true time" even in the 20th century. It is no longer used except in historical contexts, like my "Celestial Navigation in the Age of Sail" workshop.

In modern celestial navigation, always, always, always work in UT.

Frank Reed
Clockwork Mapping / ReedNavigation.com
Conanicut Island USA
Celestial Navigation in the Age of Sail, in-person at Mystic Seaport Museum in late February, or ONLINE first weekend in March.