Determine the time zone in which you are currently located. You can use any location on the globe for these calculations. Along with finding the current time zone you are in, you will need to find the time zone to which you are traveling.
Look up the time difference between these two time zones and GMT. Most time references show the number of hours plus or minus GMT for any time zone on the planet. This indicates the number of hours ahead or behind the current time in Greenwich.
Determine the direction that your destination is from your origin. You must know if you are east of your origin or west of your origin to calculate properly the time difference. Your relative location at your destination will tell whether you are ahead of or behind the time of your origin.
Calculate how many hours the change is between your origin and destination. Add the hours from GMT between your origin and destination. If the adjustment to GMT is the same sign either positive or negative, then subtract the adjustments. If however, the adjustments are of opposite signs, then add the adjustments. For example, consider the following locations and their time referenced to GMT: Washington D.C. -- GMT -- four hours, Rome, Italy -- GMT + two hours, and Moscow, Russia -- GMT + four hours. The time difference between Washington, D.C. and Rome, Italy is 4 + 2 = 6 hours between Rome, Italy and Moscow, Russia is 4 - 2 = 2 hours.
Adjust the local time by the number of hours different you are than your origin. If your location is to the west, then your time change is behind your origin, so subtract the appropriate number of hours you found from the current time to find the time at home. If you travel east on the other hand, your time change is ahead of your origin. For example, assume you depart from Washington, D.C., fly to Rome and land at 2 p.m. Washington, D.C. time is six hours behind Rome. Rome, Italy is two hours ahead of Moscow. If the time is 2 p.m. in Italy, then the time is 8 a.m. in Washington, D.C. Moscow time is two hours ahead of Rome time so it is 4 p.m. in Moscow.
