In 1884, the International Meridian conference was held in Washington, D.C. to set up a system of Time Zones. The Earth was divided into 24 zones, one for each hour of the day. Thus, each time zone spans 15 degree of longitude.
The line of the Greenwich meridian time zone (Universal Time) is used for reference, and times to the east or west can be distinguished by comparison with it. Thus, time zones to the West of the prime meridian are said to have slow time and places to the East are on fast time.
So a place 90 degree to the east of the prime meridian would be 6 hours fast, white time in the Pacific Time Zone in the United States, whose central meridian is 120 degree west is said to be 8 hours slow.