singular vs plural - Two-year program or Two-years program?

Which is correct, a "two-year" program or "two-years" program?

The difference between two expressions is the absence/presence of "s", i.e., singular or plural.

Answer

When we use counted elements as adjectives, they take a hyphen and lose the plural ending -s, because adjectives don't have plural forms in English:

Here are some examples:

a two-year program, a 3-day hike, a two-hour test, a four-year-old child, a 100-year war