佛學辭典及百科全書多云：五十六或五十七億年後，彌勒菩薩將下生娑婆世界成佛。然而，這答案極可能不正確！為什麼呢？
如眾所周知的，彌勒佛將是此娑婆世界的下一尊佛。然而，彌勒菩薩究竟將於何時出世成佛呢？
《佛祖統紀》卷三十及宣公上人《妙法蓮華經淺釋》上冊129頁提到：人壽一百歲時，釋迦牟尼佛出世。人壽由百歲減至十歲，由十歲增至八萬四千歲，再由八萬四千減至八萬歲時，彌勒佛將出世。以每百年人壽增、減一歲計算，吾人可估計釋迦牟尼及彌勒二佛出世相距年數如下：
人壽由100減至10歲：
(10010) x 100 = 9,000年
由10增至84,000歲：
(84,00010) x 100 = 8,399,000年
由84,000減至80,000歲：
(84,00080,000) x 100 = 400,000年
合計： 8,808,000年
故二佛出世相距八百八十萬八千年。此估算與莊椿錄之八百八十萬九千年只相差一千年(《佛祖統紀》卷三十)。
雖然《長阿含經》卷六、《中阿含經》卷十三，及《阿毘達磨大毘婆沙論》卷一七八亦云：人壽八萬歲彌勒出興，《賢劫經》卷七、《菩薩從兜術天降神母胎說廣普經》 (亦名《菩薩處胎經》)卷二、《大彌勒成佛經》、《佛說彌勒下生經》、《佛說彌勒來時經》，及《大智度論》卷三皆云：人壽八萬四千歲，彌勒出興；若此則釋迦牟尼及彌勒二佛出世相距將減四十萬年成為八百四十萬八千年。
《佛說處處經》云，彌勒五億七千六十萬歲後當下作佛，當以兜率天壽計算：閻浮提四百歲相當兜率天一日一夜(《俱舍論》卷十一)，以三十日為一月、十二月為一歲，彼天壽四千歲 (《長阿含經》卷二十、《起世經》卷七、《俱舍論》卷十一)，合此娑婆世界五億七千六百萬歲。
《佛說觀彌勒菩薩上生兜率天經》、《一切智光明仙人慈心因緣不食肉經》、《菩薩從兜術天降神母胎說廣普經》卷二，及《賢愚經》卷四等，皆云彌勒菩薩將於五十六億歲後出世。《丁福保佛學辭典》、《佛光佛學辭典》、《數位佛學辭典》及《中華佛教百科全書》等，亦云五十六或五十七億歲。《大毘婆沙論》卷一七八，則云五十七俱胝歲，故億應為俱胝之譯；然俱胝為千萬(《俱舍論》卷九、《丁福保佛學辭典》)。〈《大智度論》卷四及窺基大師之觀彌勒上生兜率天經贊〉卷下亦云千萬為億。今人以萬萬為億，則五億七千六百萬歲方為正確 (〈觀彌勒上生兜率天經贊〉、《彌勒上生經宗要》、《三彌勒經疏》〉卷一)。
然而菩薩不一定窮兜率天壽方下生，恰似人壽長短不一；釋迦牟尼佛雖在人壽百歲下生，卻在八十歲時入滅即為一例。《彌勒經遊意》及〈觀彌勒上生兜率天經贊〉卷上云：人壽千二百歲，釋迦始生兜率，人壽一百歲下生作佛。人壽百歲彌勒生天，人壽八萬四千歲下生成佛。可見二佛天壽不同。又《佛祖統紀》卷三十云：釋迦牟尼佛出世本在人壽一萬歲時，為觀眾生無機可度，至百歲劫末苦逼，始出乎世。因此菩薩不必窮天壽方下生。
故彌勒菩薩當於八百萬或五億七千萬年後下生成佛，而八百萬年之說似較合理。
《丁福保佛學辭典》〈兜率〉一條，正確的指出兜率天一年為人間十四萬四千年。然兜率天壽四千歲，卻錯誤的計算為人間五十七億六百萬年。


Most, if not all, Buddhist dictionaries and encyclopedias indicate that Maitreya Bodhisattva will be born and become the next Buddha in the Saha World 5.6 or 5.7 billion years later. However, this answer is most likely incorrect. Why is that?
It is well known that Maitreya Bodhisattva will be the next Buddha in this Saha World. However, the question is, when will this happen?
A Chronicle of the Buddhas and the Patriarchs (Fasc. 30) and Venerable Master Hsuan Hua’s Commentary on the
Lotus Sutra (Vol. 2, p.156) say that Shakyamuni Buddha was born when the average human life span was 100 years. The human life span will decrease from 100 years to 10 years, then increase from 10 years to 84,000 years, and then decrease from 84,000 years to 80,000 years. At that time Maitreya Bodhisattva will be born and become the next Buddha in the Saha World. As the average human life span decreases/increases by 1 year every 100 years, one can calculate how many years separate the births of Shakyamuni Buddha and Maitreya Buddha.
Life span from 100 to 10 years: (10010) x 100 = 9,000
From 10 to 84,000 years: (84,00010) x 100 = 8,399,000
From 84,000 to 80,000 years: (84,00080,000) x 100 = 400,000
Total: 8,808,000
Therefore, the births of the two Buddhas are separated by 8,808,000 years. This number differs by only 1,000 years from TL Zhuang’s estimate of 8,809,000 years (A Chronicle of the Buddhas and the Patriarchs,Fasc. 30).
While the Long Agama Sutra (Fasc. 6), the
Intermediate Agama Sutra (Fasc. 13) and the AbhidharmavibhÁçÁäÁstra (Fasc 178) also say that Maitreya Bodhisattva will be born when the average human life span is 80,000 years, at least five other Sutras and one Shastra (Bhadrakalpa Sutra (Fasc. 7),
The Sutra of the Bodhisattva’s Descent from Tushita Heaven into his Mother’s Womb (Fasc. 2),
The Sutra on Maitreya’s Achieving Buddhahood, The Sutra on the Descent of Maitreya, Sutra on the Coming of Maitreya, and MahÁprajñÁpÁramitÁäÁstra (Fasc. 3)) all say that Maitreya Bodhisattva will be born when the average human lifes pan is 84,000 years. Should this be the case, Maitreya Bodhisattva will be born 400,000 years earlier, i.e., 8,408,000 years after Shakyamuni Buddha.
However, at least one Sutra (Foshuo chuchu jing) says that Maitreya Bodhisattva will be born 570 million years later. This calculation must have been based on the lifes pan of the gods in the Tushita Heaven.
The gods in the Tushita Heaven have a life span of 4,000 years (Long Agama Sutra (Fasc. 20),
Qishi jing (Fasc. 7), AbhidharmakoäabhÁsya (Fasc. 11)). However, one day there is equivalent to 400 years on earth (AbhidharmakoäabhÁsya, Fasc. 11)! So, 100 Tushita days equals to 40,000 earth years, and 1 Tushita year is 144,000 earth years (assuming 360 days in a year.) The gods’ life span of 4,000 Tushita years is thus equivalent to 576 million earth years.
However, The Sutra of the Contemplation of Maitreya Bodhisattva’s Ascent to Tushita Heaven Spoken by the Buddha, The Sutra of the Immortal of AllWisdom’s Light Who Abstained from Eating Meat for Reasons of Kindness, The Sutra of the Bodhisattva’s Descent from Tushita Heaven into his Mother’s Womb (Fasc. 2),
The Sutra on the Wise and the Foolish (Fasc. 4), the Ding Fubao^{1} and Foguang Buddhist Dictionaries, Charles Muller’s Digital Dictionary of Buddhism and the Chinese Buddhist Encyclopedia all say that Maitreya Bodhisattva will be born 5.6 or 5.7 billion (56 or 57
yi (億)) years later. The AbhidharmavibhÁçÁäÁstra (Fasc.178), nevertheless, gives 57
koti years; so yi (億) must be the Chinese translation of
koti. One koti is 10 million (AbhidharmakoäabhÁsya Fasc.9, Ding Fubao Buddhist Dictionary). Nagarjuna Bodhisattva (MahÁprajñÁpÁramitÁäÁstra (Fasc. 4)) and Great Master Kui Ji (Praise to the Sutra of the Contemplation of Maitreya Bodhisattva’s Ascent and Birth in the Tushita Heaven (Fasc. 2)) also interprets
yi (億) as 10 million. Nowadays, however, yi (億) means 100 million. So it should be 5.76
yi (億) or 576 million earth years before the birth of Maitreya Bodhisattva (Praise to the Sutra of the Contemplation of Maitreya Bodhisattva’s Ascent to Tushita Heaven, Summary of the Sutra on the Ascent of Maitreya, Prologue on Three Sutras on Maitreya, (Fasc. 1)).
It is not for sure that all Bodhisattvas will live for 4,000 years in the Tushita Heaven, just as humans have different life spans on earth. For example, Shakyamuni Buddha entered nirvana at an age of 80 when the average human life span was supposed to be 100 years. In fact, Shakyamuni and Maitreya live different lengths of time in Tushita Heaven. Shakyamuni Buddha was born in Tushita Heaven when the average human life span was 1,200 years and was then born in the Saha World to become a Buddha when the average human life span was 100 years. Maitreya, on the other hand, was born in the Tushita Heaven when the average human life span was 100 years and will be born in the Saha World to become a Buddha when the average human lifespan is 84,000 years (Mile jing youyi彌勒經遊意,
Praise to the Sutra of the Contemplation of Maitreya Bodhisattva’s Ascent to Tushita Heaven, Fasc. 1). Besides, Shakyamuni Buddha was originally scheduled to be born on earth when the average human life span was 10,000 years. However, the conditions weren’t right, so he waited until the human life span decreased to 100 years (A Chronicle of the Buddhas and the Patriarchs, Fasc. 30). Obviously, Bodhisattvas need not to live up to 4,000 years in the Tushita Heaven.
To conclude, Maitreya Bodhisattva will be born in the Saha World to become the next Buddha either 8 or 570 million years later. The former seems to be more plausible.
^{1}Ding Fubao Buddhist Dictionary rightly gives 144,000 earth years for one Tushita year; however, it erroneously calculates 4000 Tushita years to be 5.76 billion years!
