Trim the ham of any excess fat, but leaving 1/4-inch of fat on the ham. With a sharp knife, score the fat on the top of the ham. Place the ham on a large rimmed baking sheet.

Whisk together the brown sugar, salt, black pepper, cumin, ginger, mustard powder, and cinnamon in a small bowl. Reserve 1 tablespoon of the dry rub mixture for the glaze. Season the ham with the remaining dry rub. Place plastic wrap over the ham and put it in the refrigerator for 8 to 12 hours. Allowing the seasoning to penetrate the ham overnight will give it a fuller flavor.

Preheat your charcoal grill to 300 degrees F for indirect heat.

Place the baking sheet with the ham on the grill grates, with the ham flat side down. Cover the grill and let the ham cook for 2 hours.

Heat the butter, thyme, honey, vinegar, and reserved dry rub together in a medium saucepan until the butter melts. Stir together until smooth. Brush the ham with the glaze, cover with grill lid, and grill for 30 minutes more, basting the ham every 10 minutes with the glaze. Remove to a platter, let rest for 30 minutes, then carve and serve.