Refer to figure.
The Air Almanac includes tables that give the Local Mean Time (LMT) of sunrise and sunset and the LMT of the start of morning civil twilight and the end of evening twilight. The times given in the Air Almanac are calculated for the Greenwich meridian, but can be taken as LMT at all longitudes.

The time of sunrise and sunset varies with latitude and time of the year. Higher latitudes experience longer days in the summer and shorter days in the winter, because of the inclination of the Earth's axis of rotation to the ecliptic.

The morning civil twilight starts when the centre of the Sun is 6^o below the sensible horizon and ends at sunrise.

The evening civil twilight starts at sunset and ends when the centre of the Sun is 6^o below the sensible horizon.

For this question:

• Use the upper "SUNSET" table to find the sunset LMT (the start of the evening civil twilight) for 11th October interpolating for N23° 24' (between N20° - N30°): 17:38.
• Use the lower "EVENING CIVIL TWILIGHT" table to find the end of the evening civil twilight LMT (when the centre of the Sun is 6^o below the sensible horizon) for 11th October interpolating for N23° 24' (between N20° - N30°): 18:01.
• The duration of the evening civil twilight at Guangzhou Balyon Airport is between 17:38 and 18:01. Thus, it lasts 23 minutes.