Introduction
Kala is Time, personified. In the Uttara Kanda he comes to Rama with a message from Brahma.
This article primarily follows the Valmiki Ramayana. Where a well-known detail comes from a later retelling or tradition, the article identifies it.
The message
Kala appeared in Ayodhya as an ascetic and asked to speak with Rama in private, on the condition that anyone who interrupted would be put to death. He told Rama that his time on earth was complete.
The interruption
Rama set Lakshmana at the door. When the sage Durvasa demanded entry, Lakshmana had to choose between interrupting Rama and letting Durvasa curse Ayodhya. He chose to interrupt, and Rama had to keep his word.