Question

Who is kaal? What is kaal?

Answer

In a philosophical sense, KAla ("kaal") is inclusive of but not limited to Time. When there is something or nothing in the universe, the KAla is always present. It's eternal God. Other things change with respect to it. Hence, because it gives the value of change (duration), it has to remain unchanged!

There are below references of the term KAla ("kaal") in Bhagavad Gita:

  1. Eternal -- For those who intend to gauge it, it's infinity

    [BG 10.30] — Among Daityas I am Prahlāda, among calculators I am KAla, among beasts I am the lion, and among birds I am Garuḍa.

  2. Destroyer -- It destroys everything, hence changes everything; Here "destruction" should not be taken always into literal sense; e.g. present 1 second will be destroyed in 1 second

    [BG 11:32] Sri BhagawAna told: I am the expanded KAla for the destruction of the loka(world); For hunting people here, I am activated; Except you (PAndava), all the soldiers situated in the opposite sides will not exist.

"Kaal" with respect to various aspects of time, is described in Mokshadharma Parva:

The Rishis, measuring time, have named particular portions by particular names. Five and ten winks of the eye make what is called a Kashtha. Thirty Kashthas would make what is called a Kala. Thirty Kalas, with the tenth part of a Kala added, make what is known as a Muhurta. Thirty Muhurtas make up one day and night. Thirty days and nights are called a month, and twelve months are called a year. Persons conversant with mathematical science say that a year is made up of two ayanas (dependent on sun's motion), viz., the northern and the southern. The sun makes the day and the night for the world of man. The night is for the sleep of all living creatures, and the day is for the doing of action.

There is more to it, which is discussed with details in below answer:
What is the duration (Timeline) of the four “Yugas”?

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