How many years are between 1988 and 1999?

That depends on exactly what you mean by "between": if you want the number of whole years that are after 1988 but before 1999, that's 10 (1989, 90, 91, 92, 93, 94, 95, 96, 97, 98). If you want the number of years between, say, April 5, 1988, and April 5, 1999, that's 11.