diff --git a/lib/urwis/engine.test.ts b/lib/urwis/engine.test.ts
new file mode 100644
index 0000000..c5c7098
--- /dev/null
+++ b/lib/urwis/engine.test.ts
@@ -0,0 +1,21 @@
+import { test, describe } from 'node:test';
+import * as assert from 'node:assert';
+import { calculateDecay } from './engine';
+
+describe('calculateDecay', () => {
+  describe('Zero Bound', () => {
+    test('bounds the result to 0 when a large amount of time has passed', () => {
+      // 1000000 seconds is large enough to decay any small value to 0, regardless of the exact rate
+      assert.strictEqual(calculateDecay(10, 1000000), 0);
+    });
+
+    test('returns 0 when initial value is 0', () => {
+      assert.strictEqual(calculateDecay(0, 10), 0);
+    });
+
+    test('returns 0 when initial value is negative', () => {
+      // Bounding should ensure negative values are clamped to 0
+      assert.strictEqual(calculateDecay(-10, 10), 0);
+    });
+  });
+});