Prime Meridian Day a.k.a. Time Truly is a Relative Thing

On November 1, 1884, representatives from 25 countries met in Washington, D.C., "to fix a meridian proper to be employed as a common zero of longitude and standard of time reckoning throughout the globe", in other words in order to establish an internationally agreed upon 0 longitude and to use that as a zero point for also establishing a worldwide time standard. Twenty-two of the 25 represented countries agreed that the prime meridian should be established in Greenwich, England. This established the standard Greenwich Mean Time (GMT), which is now Universal Time (UTC).

Establishing universal time, let alone a longitudinal "0" location to attach that time to, is a human construct. These constructs are important because they help us to communicate/identify our location (in time and space ; ) in a way that we can specifically define. Establishing a prime meridian was especially important for mapping, navigation, travel, and scheduling.

Science uses many "constructs" to let us explore, research, identify, and communicate about the world. A good example is types of measurement (weights, distances, volumes...), which we have largely standardized, but even the differences in using inches vs. centimeters, miles vs. kilometers, and gallons vs. liters causes confusion, etc..
Establishing a prime meridian/ universal time was and is also important culturally because it demonstrates our global interconnected-ness and gives us common ground.

Fun Fact: Knowing where you are on earth (longitudinally) means that you can figure out what time it is, and visa-versa: every 15° longitude equals an hour difference in time.