When we consider 15 meridian on a world map or globe and count them in an eastward direction starting with Greenwich meridian (0), we find that the time of this meridian is

The Earth completes a full rotation of 360° in 24 hours, which equates to 15° of longitude per hour [c2, c3]. The Greenwich Meridian (0°) serves as the reference point for World Time or Greenwich Mean Time (GMT) [c1]. Because the Earth rotates from west to east, places located to the east of the Prime Meridian see the sun earlier and are ahead in time [c2, t4]. Specifically, for every 15° moved eastward, the local time is advanced by one hour [c2, t1]. Therefore, at the 15th meridian east, the time is exactly 1 hour fast compared to Greenwich [c3, t6]. Conversely, moving westward would result in time being retarded or 'slow' [c2]. This mathematical relationship (15° = 1 hour) is the basis for the global time zone system [c5, t8].

Sources

1. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 18: Latitudes and Longitudes > 18.2. Longitude or Meridian > p. 243
2. [2] Certificate Physical and Human Geography , GC Leong (Oxford University press 3rd ed.) > Chapter 2: The Earth's Crust > Longitude and Time > p. 11
3. [3] Exploring Society:India and Beyond. Social Science-Class VI . NCERT(Revised ed 2025) > Chapter 1: Locating Places on the Earth > Understanding Time Zones > p. 20
4. [4] Exploring Society:India and Beyond. Social Science-Class VI . NCERT(Revised ed 2025) > Chapter 1: Locating Places on the Earth > LET'S EXPLORE > p. 21