How Time Zones Work
Each time zone is then theoretically 15 degrees wide, corresponding to a one-hour difference in mean solar time. The shape of time zones is changed, in practice, to match internal and international borders. Civil time changes by one hour forward and backward respectively for every 15 degrees east or west of the Greenwich Meridian. One would need to divide the longitude, in degrees, by 15 to find the appropriate time zone, in hours. For example:
At 150 degrees west (or 150° W) longitude, the time should be 150 degrees divided by 15 degrees = 10 hours behind Coordinated Universal Time (UTC/GMT) (or UTC-10).
At 75 degrees east (or 75° E) longitude, the time would be 75 degrees divided by 15 degrees = 5 hours ahead of UTC/GMT (or UTC+5).