1. Preheat broiler to high.
2. Bring 1 1/2 cups water to a boil in a medium saucepan. Stir in couscous; cover pan. Remove from heat; let couscous mixture stand, covered, for 5 minutes. Uncover pan; fluff couscous with a fork. Combine buttermilk, 1 1/2 teaspoons cumin, 1/4 teaspoon salt, and 1/4 teaspoon black pepper in a large bowl, stirring with a whisk. Add couscous, walnuts, cilantro, dates, and green onion; stir mixture gently to combine.
3. Drizzle olive oil evenly over chicken thighs; sprinkle both sides of chicken with remaining 1/2 teaspoon cumin, remaining 1/8 teaspoon salt, remaining 1/8 teaspoon pepper, and garlic. Arrange chicken in a single layer in a large ovenproof skillet. Broil chicken for 5 minutes; turn chicken over. Broil an additional 3 minutes or until done. Remove chicken from pan; let stand 5 minutes. Pour pan juices into couscous mixture; toss gently to combine. Spoon about 1 cup couscous mixture onto each of 6 plates. Slice chicken crosswise; divide chicken evenly over couscous.