Buddha's Dispensation
Buddha's Dispensation (Buddha-sasana) is the time period which Dharma (Pali Canon) exist and follow. It is totally 5000 years after the Buddha's Parinirvana (pass away or "Skandha Parinirvana"). There are three types of Parinirvana. These are "Klesha Parinirwana", "Skandha Parinirvana" and "Dhatu Parinirvana".
Dhatu Parinirvana will occur the end of the Buddha's Dispensation (totally 5000 years after the Buddha's Skandha Parinirvana). It happens according to the Buddha's wish even though he doesn't exist. All Buddha's Relics in the Ruwanwelisaya (the place where a large amount of Buddha's Relics exist) in Sri Lanka will join together and make a picture of a Buddha in the sky above Ruwanwelisaya. Then those relics and all Relics associated with Buddha in the human, Naga, heaven and Brahma worlds will came to the sky above the Vajrasana (diamond throne) in 'Bodh Gaya' of India and will make a complete picture of Buddha which will preach Dharma for the last time. This will not be a normal preaching like in the 'Sutta Pitaka' in Pali Canon. It will be a 'Samukkansika mode of preaching' extensively on 'Four Noble Truths' which preach with Buddha's super-normal abilities. This is a very long (containing many hundred thousand words) preaching, done in high speed but every one can understand each word clearly. With this preaching, many beings realize Nirvana. Then Vajrasana will come to the sky from the earth and all Buddha's Relics and Vajrasana will be completely burned out but without any ash.
After this 'Dhatu Parinirvana' and when the last Sotāpanna human passed away from the earth it is said to be the end of Buddha's Dispensation. Before the end of Buddha's dispensation any one who correctly follows Buddha's words is said to be able realize the Nirvana. But after the Buddha's Dispensation it seems it is impossible since no one has the energy to correctly follow Buddha's words. But if any great being correctly follows Buddha's words even after the Buddha's Dispensation it is said to be able to realize the reality (Nirvana).