How many years are between 900 bce and620 ce?

There are two complications to this question: the fact that the year 0 did not happen, and also the fact that people frequently misunderstand "between" to include the ends of the range. From 899BC to 1BC is 899 years. From 1 to 619 AD is 619 years. 899 + 619 = 1518 years between -- that is, years which are neither in 900 BC nor in 620 AD.