can someone explain something to me about the international date line when travelling by air?

how is it that is travelling from west to east by air the journey you lose a day when crossing the internatoional date line but at the same time you arrive 2 days later.

i`ve been looking at flights and if you leave on say the 1st you arrive on the 3rd how can this happen if you lose a day

It all depends on the time you leave and it's actually when you're traveling from east to west that you will lose a day when you cross the dateline. For example, Hong Kong is 16 hours ahead of San Francisco or the west coast, so if you leave on the 1st at 9:00pm, it's already 1:00pm on the 2nd in Hong Kong when you depart, and a flight to Hong Kong is typically 15 hrs, so if you add 15 hrs. to 1:00pm on the 2nd, it will be 4:00am on the 3rd when you arrive in Hong Kong. It is confusing, but if you leave on a flight shortly after midnight you don't lose a day and will arrive the next calendar day early in morning, I'll let you do the math on that one. The good thing is that you get your day back when go home, as a flight from Hong Kong to the west coast will typically arrive 3-4 hrs. before the time you left on the same day.

Lets say the flying time is12 hours and the flight leaves at 10am on Monday the 1st.

You would expect to land at 10pm on Monday the 1st and indeed at the time you land it will be 10pm on Monday the 1st in the place you flew out of.

However when you left at 10am on Monday the 1st - the time in the country you are flying to is NOT 10am on the first its 5am on Tuesday the 2nd. So when you land 12 hours later it will be 5pm on Tuesday the 2nd.

Therefore having left at 10am on Monday 1st you will land at 5pm on Tuesday.

Have a look at this website to see what the time is in the Cities you will be travelling from and to and add the flight time to both so you can see where you will "lose" a day.

http://www.timeanddate.com/worldclock/

For instance if you are flying from LA to Australia most flights leave late in the evening, so you might leave on Monday evening at 10pm and fly for 15 hours and arrive at 8am Wednesday in Sydney - a better example of "losing a day"

The International Date Line is a very weird but very cool thing. Its a man-made line and is the point on the earth's surface at which one day ends and another day begins, ie one date ends and the next date begins. The line runs down the middle of the Pacific Ocean, because when the Date Line was first created by scientists in the 1700's (or thereabouts), nobody except explorers crossed the Pacific. But now with jet air travel, millions of people a day cross the Date Line when traveling from Asia to North America, from Asia to South America, from Australia or New Zealand to North or South America, and between certain Pacific Island groups - eg from Fiji to Hawaii, or from Fiji to Tahiti.

Here's a simple explaination for how it works -

Flying from Sydney to Honolulu takes 9 hours. But because you cross the Date Line from west to east, you gain 1 day. So your flying time is 9 hours sitting on the plane, but when you cross the Date Line, your watch goes back 24 hours, since you just crossed back into the previous date. So the flying time on your watch is 9 hours, minus 24. If you departed Sydney at 5pm on the 1st, you arrive at Honolulu 16 hours earlier (24 - 9 = 16). So you arrive in Honolulu at 1am on the 1st! Therefore, you have two 1st's of that month!

Going the other way, you depart Honolulu at 5pm on the 1st, but at the Date Line, your watch goes forward 24 hours, because you are crossing over into the next date. Once again you're sitting on the plane for 9 hours, but according to your watch, you have traveled 9 hours plus 24. So you arrive in Sydney 33 hours later at 2am on the 3rd! (9 + 24 = 33)! For you and the other passengers, the 2nd of that month didn't exist!

Weird, huh???