One thing that I wonder is why some flights from Asia to North America arrive a day before arrival.

This is an example of one. CX872 departs HKG saturday 12:55 AM, SFO arrival 9:10 PM on a friday.

Don't really understand why this happens for some flights, would like to understand more about how time zones work. There are some Asia-North American flights that depart around 11PM and arrive North America next this, but this is a different exception.

The key to understanding time zones is that at the same moment, different places on Earth have different times. This because the Earth spins once every 24 hours*. This means a given location faces the sun at noon every 24 hours. Since it is the datum for our time system, let's say it is noon in Greenwich. This is "UTC time"**. At the same time as it is noon in Greenwich, it is 2000 in HKG and 0400 in SFO***. Same actual moment, different times.

In your example;
- 0055 Saturday in HKG is 1655 Friday UTC.
- 1655 Friday UTC is 0855 Friday Pacific Time.

This means that while the aircraft departs HKG on Saturday morning, at the same time it is Friday morning in SFO. It then flies for around 12 hours and lands on Friday evening in SFO. At the time it lands it is early afternoon Saturday in HKG.

In order to avoid confusion, all operational times in aviation, such as flight plans, NOTAMS, and weather reports are in UTC. Only passenger schedules are in local time.




* We could nitpick with sidereal time but let's keep it simple.
** UTC time is not quite the same as GMT, but for practical purposes it is.
*** Keeping daylight savings time out of the discussion for simplicity.

Hello,

Historically, you are not the first person confused by this apparent discontinuity of time.
Phileas Fogg, main character of Jules Verne's novel
https://en.m.wikipedia.org/wiki/Around_the_World_in_Eighty_Days feared to have arrived one day late to win the bet of surrounding the Earth in 80 days, but had failed to consider the date line.

Cheers,
Hendric

3D101CA, I would like to emphasize the source mentioned above by kalvado (Reply # 2) concerning the International Date Line to help explain why some eastbound flights from Asia to the United States arrive a day prior to departure.

As you travel from west to east, the local time becomes progressively later. The IDL was created to help normalize the time/date relationship and it has an impact, as in the example you provided HKG-SFO, on many transpacific operations.

Traveling from west to east, as you cross the IDL, the date reverts to the prior day, so depending on what time you depart from an airport in a city in Asia or the Far East, it is generally the previous day in the United States/Canada/Mexico and therefore, based on the flight time, it may continue to be the previous day when arriving at the destination.

The reverse situation is true for westbound transpacific flights. As the aircraft crosses the IDL, the date automatically advances to the next day; in essence, you lose a day traveling westbound and gain a day going eastbound. Even on relatively short flights crossing the Pacific; i.e., Honolulu to Tokyo, 8 hours 20 minutes; departure from HNL at 1240 (L), arrival NRT at 1600 (L), the arrival will be the following day.

This is not necessarily true for flights that cross the Atlantic. Compare departure/arrival dates and times for flights of similar length between destinations in various theatres; for example HKG-SFO versus JFK-DXB (13 hours) for interesting observations.

e38

GalaxyFlyer wrote:

On a blog written by a retired US foreign service officer, he writes a funny story about the Japanese staging a Christmas party for the diplomatic community in a small nation on Dec 7th. The whispered conversation about the date finally was presented the the host who innocently said he realized the Day of Infamy, Dec 8th. The US senior officer present gently mentioned in Hawaii, it was Dec 7th. Oops, profound apologies all around as the Japanese ambassador was deeply embarrassed.

LOL that's a good one - by just a hair of early morning hours due to 19-hour time difference, the attack is noted in Japanese history as Dec 8th.

Further to info posted, this is also why most airlines adhere to UTC time for internal purposes. Keeps things uniform.

Was always fun explaining to new hires that tonight’s flight is actually on the schedule as tomorrows, and so on. But could be a bit challenging to understand, especially when converting to local time

How to make BIG BUCKS in the aviation business using the time zones!
Interesting days work for a KE B777 04 Sep ‘22 (from FlightRadar 24 data)

DATE FROM TO FLIGHT STD ATD STA STATUS
TIME
05 Sep 2022 Halifax (YHZ) Anchorage (ANC) KE288 5:05 PM 4:54 PM 7:45 PM Estimated 7:07 PM KML CSV Live

05 Sep 2022 Chicago (ORD) Halifax (YHZ) KE288 2:27 10:30 AM 11:05 AM 3:35 PM Landed 3:32 PM KML CSV Play

04 Sep 2022 Anchorage (ANC)Chicago (ORD) KE287 5:24 11:30 PM 12:28 AM 8:30 AM Landed 8:52 AM KML CSV Play

05 Sep 2022 Seoul (ICN) Anchorage (ANC) KE287 7:35 7:25 AM 8:45 AM 10:25 PM Landed 11:20 PM KML CSV Play

04 Sep 2022 Penang (PEN) Seoul (ICN) KE350 6:04 11:25 PM 11:25 PM 6:45 AM Landed 6:28 AM KML CSV Play

04 Sep 2022 Jakarta (CGK) Penang (PEN) KE349 2:00 6:00 PM 6:23 PM 9:30 PM Landed 9:23 PM KML CSV Play

04 Sep 2022 Seoul (ICN) Jakarta (CGK) KE349 7:00 11:00 AM 11:27 AM 4:00 PM Landed 4:27 PM KML CSV Play

04 Sep 2022 Hong Kong (HKG) Seoul (ICN) KE314 3:07 3:20 AM 3:44 AM 7:50 AM Landed 7:51 AM KML CSV Play

03 Sep 2022 Seoul (ICN) Hong Kong (HKG) KE313 3:35 10:25 PM 11:02 PM 1:10 AM Landed 1:37 AM KML CSV Play


Beat the clock into ANC at 23:20 Sep 04.
Looking back at day 04 Sep at 23:59:59 in ANC we see:

Sep 04 Flight times: 1:37, 3:07, 7:00, 2:00, 6:04 KE350 0:35 04 Sep +5:47 05 Sep,
7:35 KE287 05 Sep, = 15.19
Sep 04 Ground time: 2:07, 3:36, 1:56, 2:02, 2:17 05 Sep, 0:40 = 10:21
25.40 hrs. on 04 Sep
while doing 13:22 air time and 2:17 ground stop (15:39 total) of Sep 05 work on the side!
Bean counters dream for calendar entry 04 Sep '22.