diff --git a/src/calculator.ts b/src/calculator.ts
index 1234567..abcdefg 100644
--- a/src/calculator.ts
+++ b/src/calculator.ts
@@ -1,6 +1,12 @@
 class Calculator {
+  private history: string[] = [];
+
   add(a: number, b: number): number {
-    return a + b;
+    const result = a + b;
+    this.logOperation(`add(${a}, ${b}) = ${result}`);
+    return result;
   }
 
   subtract(a: number, b: number): number {
@@ -8,10 +14,16 @@ class Calculator {
   }
 
   multiply(a: number, b: number): number {
-    return a * b;
+    const result = a * b;
+    this.logOperation(`multiply(${a}, ${b}) = ${result}`);
+    return result;
   }
 
   divide(a: number, b: number): number {
-    return a / b;
+    if (b === 0) {
+      throw new Error('Division by zero is not allowed');
+    }
+    return a / b;
   }
+
+  private logOperation(operation: string): void {
+    this.history.push(`${new Date().toISOString()}: ${operation}`);
+  }
 }

 export default Calculator;

diff --git a/src/utils.ts b/src/utils.ts
index 7654321..1234abc 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -1,7 +1,11 @@
 /**
  * Format a number as currency
  */
-export function formatCurrency(amount: number, currency: string = 'USD'): string {
+export function formatCurrency(
+  amount: number,
+  currency: string = 'USD',
+  locale: string = 'en-US'
+): string {
   return new Intl.NumberFormat('en-US', {
     style: 'currency',
     currency: currency,