Kālá is a Sanskrit word for "time". It denotes a fixed or right point in time (compare rtu, kairos). It is also the name of a deity, in which sense it is not always distinguishable from kāla meaning "black".
kāla 1 means "black, of a dark colour, dark-blue ..." and has a feminine form ending in ī – kālī – as mentioned in Pāṇini 4-1, 42.
kāla 2 means "a fixed or right point of time, a space of time, time ... destiny, fate ... death" and has a feminine form (found at the end of compounds) ending in ā, as mentioned in the ṛgveda Prātiśākhya.
According to Monier-Williams, kāla 2 is from the verbal root kal "to calculate", while the root of kāla 1 is uncertain, though possibly the same.
As applied to gods and goddesses in works such as the Devī Māhātmya and the Skanda Purāṇa, kāla 1 and kāla 2 are not readily distinguishable. Thus Wendy Doniger, translating a conversation between Śiva and Pārvatī from the Skanda Purāṇa, says Mahākāla may mean " 'the Great Death' ... or 'the Great Black One' ". And Swāmī Jagadīśvarānanda, a Hindu translator of the Devī Māhātmya, renders the feminine compound kāla-rātri (where rātri means "night") as "dark night of periodic dissolution".
In Borobudur, the gate to the stairs is adorned with a giant head, making the gate look like the open mouth of the giant. Many other gates in Javanese traditional buildings have this kind of ornament. Perhaps the most detailed Kala Face in Java is on the south side of Candi Kalasan.