“Time is the continuous progression of existence that occurs in an apparently irreversible succession from the past, through the present, and into the future.” Wikipedia

Time is one of the seven fundamental physical quantities in both the International System of Units (SI) and International System of Quantities.

A second is the SI unit of time, defined as exactly 9,192,631,770 cycles of the radiation that corresponds to the transition between two hyperfine levels of the ground state of the cesium-133 atom.

What is time, and why does it exist? etc., those aren’t computer science problems, so I don’t know and don’t care either.

Before radioactive matter, we used solar, earth rotation, and other stuff to calculate the time. But today we need time to be more precise, and also based on location.

1. Satellite Time

What it is:

The time kept by each GPS satellite’s atomic clock.

Why it exists:

A satellite must timestamp every signal it sends.

But its clock drifts slightly due to physics (temperature, relativity, imperfections).

So each satellite has its own imperfect time, which must be corrected.

"The raw clock inside the satellite."

2. GPS Time

What it is:

A global, continuous time scale used by the entire GPS system.

Started on **January 6, 1980**.

Why it exists:

• All satellites need a shared master time; otherwise, distances cannot be computed.
• GPS Time is smooth (no leap seconds), making math simple.
• Satellites broadcast correction data so receivers convert **satellite time → GPS time**.

"The official time of the GPS network."