When will Maitreya Buddha come according to the Manimekalai? According to Buddhism, Maitreya Buddha is a Buddha who will come some time in the future.  Some sources say he will come hundreds of thousands of years from now, but the Manimekalai, a 6th century Buddhist epic poem written in the language Tamil, seems to suggest an earlier timeframe.  Here is what chapter 12 of the Manimekalai says:


  That dharma, people in this world do not know.  But within the circuit of this universe, the devas understand it and at their request the Deva will come down again to this world from the Tushita Heaven in the year 1616. Then everyone in this world will feel impelled to practice the doctrine or mercy.


My question is, what is this year "1616" that the Manimekalai identifies as the year when Maitreya Buddha will come?  What calendar is it using?  It's clearly not the Gregorian calendar, since that wasn't in vogue in 6th century India.
Let's bear in mind that according to the sources referenced in the Wikipedia page Manimekalai, this book was written either before the 5th century CE, in the 6th century CE or latest 950 CE according to different experts.

Anne E. Monius is a Professor of South Asian Religions at Harvard University and "a historian of religion specializing in the religious traditions of India. Her research interests lie in examining the practices and products of literary culture to reconstruct the history of religions in South Asia." She wrote the book "Imagining a Place for Buddhism: Literary Culture and Religious Community in Tamil-Speaking South India".

She talks about the book Manimekalai and 1616 prediction in this book, for which you can find the excerpt here. Please pay attention to the last paragraph on page 94, and the next two pages.

Her best guess is that since the Buddhist calendar usually starts counting from the final passing (parinirvana) of the Buddha (also see this answer), this refers to the year 1616 on that calendar. She is not sure about how this number was derived.

From the Wikipedia page on the Buddha, the date of the passing away of the Buddha is given as either 483 BCE or 400 BCE. That puts 1616 at 1133 CE or 1216 CE. This is definitely centuries after the authorship of the Manimekalai. So, that's quite sensible. Prof. Monius stated that this is more specific and substantially earlier than the estimates given in the Pali sources, for the coming of the next Buddha.

According to accesstoinsight.org's page on the Chronology of Theravada Buddhism:


  Year 1 of the Buddhist Era calendar is the year of the Buddha's
  Parinibbana (death and final release), which occurred in the Buddha's
  eightieth year (480 BCE according to the "historical" timeline; 544
  BCE by tradition).


If we use 544 BCE as the year of the Buddha's parinirvana, then 1616 would be 1072 CE, which is still centuries after the authorship of the Manimekalai (if we assume that it was authored in the 6th century CE).

Also interesting is this excerpt from the book by Monius (on pages 95 - 96):


  In fact, if one looks beyond, or more appropriately before, the Pali
  literature that obviously concerns itself with the future Buddha,
  scattered bits of evidence can be found to suggest that the
  Manimekalai was not alone in the sixth century in imagining the
  arrival of a living Buddha within centuries rather than billions of
  years. In the various renditions of the life of Buddhaghosa, for
  example, it is reported that the monks of the Mahavihara greeted the
  composition of the Visuddhimagga by crying out to its author, "Without
  doubt he is Metteyya!" (nissamsayam sa metteyo). Although such a
  statement might easily be interpreted, on the one hand, as hyperbolic
  praise, likening only rhetorically the genius of Buddhaghosa to
  the wisdom of the future Buddha, on the other hand, such an
  identification might well depend on the expectation, like that of the
  ascetics confused by the miraculous events accompanying Aputtiran's
  rebirth in the Manimekalai (xv.23-35), that Metteyya's arrival on earth
  can happen at any moment. Such scattered phrases are certainly
  difficult to evaluate. Much more interesting and potentially useful
  for the consideration of the Manimekalai's vision of the future, given
  the central place of the wondrous almsbowl, Amutacurapi, in the text,
  is a story told by the fifth-century Chinese pilgrim to India and Sri
  Lanka, Faxian (Fa-hsien), concerning the begging bowl of the Buddha
  and the coming of Metteyya/Maitreya to earth.


You can read on in the same section, the story of Faxian, and how he obtained the story of the Buddha's almsbowl from an Indian monk.

Monius writes further on pages 96 - 97:


  Faxian's story of the Buddha's almsbowl and its intimate connection to
  the  coming of Maitreya provides a potentially enlightening parallel
  to the narrative of the bowl and the coming of the Buddha found in the
  Manimekalai. Such a parallel, especially given the relative proximity
  to the time and place of the Manimekalai's composition, of Faxian and
  the Indian monk who tells the story at the Sri Lankan Mahavihara, may
  help to illumine the Tamil text's vision of the future and the
  community to form around the coming Buddha.


Based on this, I would guess that the Manimekalai is a literary work of fiction, based on Faxian's story that came not long before the authorship of the Manimekalai. While it is unknown how Manimekalai's author derived the number "1616", we can speculate that he intended his readers to look forward to the coming of the future Buddha within a few centuries, perhaps to motivate the Tamil-speaking Buddhist community at the time, and also to kindle the interest of the non-Buddhist Tamil-speaking people, in Buddhism.