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 those are "Klesha Parinirwana", "Skandha Parinirvana" and "Dhatu Parinirvana".
Buddha's Dispensation began in 1500
It is neither because it gives us no information on when it started. The system thinks the context of 5000 years is when it began from what I can figure out