Finding the time taken by a flight I came across a question today which is as follows:

A Flight takes off at 2 A.M from northeast direction and travels for 11 hours to reach the destination, which is in northwest direction. Given the latitude and longitude of source and destination. Find the local time of destination when the flight reaches there?

I tried it and added 2 am to 11 hours which gave "1pm". But , we also have to take in consideration the time zones , so since it travels east to west , there has to be some time subtracted. How can I figure out that time . 
Sorry if this was a duplicate question.
Thanks for your help.
 A: In reality, the answer that follows does have exceptions, as there are peculiarities in the world's timezone set-up, but the question implies that we don't need to consider these. In the real world, you would obviously need to consider the longitude of the capitals of the country/countries the source and destination are in. Even then, there are peculiarities: you would need to familiarise yourself with the timezone set-up of the country/countries you're dealing with.  
Let S be the degrees component of the source and D of the destination.  
If both source and destination are in the western hemisphere, let S = -S and D = -D if they aren't already prefixed by '-'. Then the time (in hours) you need to subtract is $$\lfloor \frac {|D|}{15} \rfloor - \lfloor \frac {|S|}{15} \rfloor.$$  
If the source and destination are in the eastern hemisphere, the time (in hours) you need to subtract is $$\lfloor \frac {S}{15} \rfloor - \lfloor \frac {D}{15} \rfloor.$$  
If the source is in the eastern hemisphere and the destination in the western hemisphere, the time (in hours) you need to subtract is $$\lfloor \frac {S}{15} \rfloor + | \lfloor \frac {D}{15} \rfloor |.$$  
This is because each timezone has a width of 15 degrees longitude.
