Who is kaal? What is kaal? What does vedas say about this vedic god? Is he vedic god? Is kaal different from Yamadeva?
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:
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.
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”?
As per Srimad Bhagavatham,
Kaalohi bhagavaan karta, satatam sutatam sukha-dukha yo ho;
naraanaam para tantraanaam janma punya papanu-yogitha-ha!
Time itself is Bhagavan, THE GOD. As per Vedam and Vedam recites Lord Srimann Narayana is Maha Kaaludu, Kaala purusha, Kaala Swarupa, Kaal Atmaka. HE himself is Maha Kaala. He is Creator, Responsible person and director of Time. With the Times(Maha Kaala), Living entities or Living beings will experiences the Happiness or Sadness(Good Time or Bad Time) based on their previous birth/Janma Karmas(Punya or paapa).
The Bhagavan has given some duties or responsibilities and Powers for HIS Subordinate deities to perform and rule as HIS Order. Among various deities Lord Yamadeva is one subordinate Deity of Lord Kaala Purusha.
Kaala is Time, represented by the Kaala Chakra, worshipped independently or Yama as a presiding deity. The text Yoga Vashishtha, a conversation between Maharishi Vashishtha and Shri Rama, has multiple references on Kaala providing beautiful insights:
• Time cannot be analyzed; for however much it is divided it survives indestructible. [1.23]
• There is another aspect of this time, the end of action (kritanta), according to the law of nature (niyati). [1.25.6-7]
• The world is like a potter’s wheel: the wheel looks as if it stands still, though it revolves at a terrific speed. [1.27]
• Just as space does not have a fixed span, time does not have a fixed span either. Just as the world and its creation are mere appearances, a moment and an epoch are also imaginary. [3.20]
• Infinite consciousness held in itself the notion of a unit of time equal to one-millionth of the twinkling of an eye: and from this evolved the time- scale right upto an epoch consisting of several revolutions of the four ages, which is the life-span of one cosmic creation. Infinite conscious- ness itself is uninvolved in these, for it is devoid of rising and setting (which are essential to all time-scales), and it devoid of a beginning, middle and end. [3.61]
Reference used: http://arxiv.org/abs/physics/9903010