Local times zones to GMT

[JBUK]

Hi, I'm building a spreadsheet to generate Aircraft Flight Itineraries. The problem I'm having is calculating arrival times.
The departure time and length of flight are both known, however, departure and arrival points are usually in different time zones and occasionally cross the international date line.

Example:
FLT TIME GMT TIME ZONE LOCAL TIME
Dep London 7:00 12:00 GMT 12:00
Arr New York 19:00 -5 14:00

or

Dep Hong Kong 10:00 05:00 +8 13:00
Arr Seattle 15:00 -8 07:00

The starting point is Local time at the departure which then needs to be converted to GMT (this is the part I can’t do), from here the arrival time in GMT is calculated (no problem) then calculate the local arrival time (can’t do). The format for time the time zone I would like to post as follows (if possible): -3,-2,-1,GMT,+1,+2,+3,+4,+4:30,+5,+5:30...etc. I have this set as a drop down menu using 'data validation'.

I'm a novice using Excel and would appreciate any help; I hope I'm not biting off more than I can chew..

Kind regards

 

[mbb4331]

Subtracting one date/time from another gives you a difference in days. Divide that difference by 24 to get hours.

Ex. To convert Pacific Time (1:30pm) to Eastern time (4:30pm), add (3/24) to Pacific Time.