Add addHalf()/subHalf() functions

src/Carbon.php

@@ -27,6 +27,26 @@ class Carbon extends CarbonBase
        }
     }
 
+    public function addHalf(): self
+    {
+        return $this->addHalves(1);
+    }
+
+    public function addHalves(int $unit): self
+    {
+        return $this->addQuarters(2*$unit);
+    }
+
+    public function subHalf(): self
+    {
+        return $this->subHalves(1);
+    }
+
+    public function subHalves(int $unit): self
+    {
+        return $this->subQuarters(2*$unit);
+    }
+
     /**
      * Modify to the first occurrence of a given day of the week
      * in the current quarter. If no dayOfWeek is provided, modify to the