"Zulu" time, more commonly known as "GMT" (Greenwich Mean Time) before 1972, is a time at the Zero Meridian. Currently, it's referred to as Coordinated Universal Time or Universal Time Coordinated (UTC). It is a coordinated time scale, maintained by the Bureau International des Poids et Mesures (BIPM). It is also known as "Z time" or "Zulu Time."
Our natural concept of time is linked to the rotation of the earth, and we define the length of the day as the 24 hours it takes (on average) the Earth to spin once on its axis.