If the time at London (0 degrees longitude) is 2 pm. Then the time be at 90 degrees west will be 8 am.
Explanation:
The time difference between two longitudes is 4 minute. And we know when we cross prime meridian and go towards west from east the time decreases. If the time at Greenwich (0 degree longitude) is 2 pm. That means the time in 90 degree west will be 8 am, 6 hours less from the time of Greenwich Mean Time. (90*4=360 minutes). 60 minutes= 1 hour. Then (360/60=6).
When we cross International Date Line from west to east one day added to our calendar and when we cross it from east to west one day reduced from calendar. International Date Line is 180 degree longitude.
