Fix arithmetic with UTC timezone objects

src/addDays.ts

@@ -16,7 +16,7 @@ export function addDays(dateObj: Date, days: number, timeZone?: TimeZone | strin
     const d = new Date(dateObj.getTime());
 
     // Handle UTC calculation
-    if (isUTC(timeZone)) {
+    if (isUTC(zoneName)) {
       d.setUTCDate(d.getUTCDate() + days);
       return d;
     }

src/addMonths.ts

@@ -16,7 +16,7 @@ export function addMonths(dateObj: Date, months: number, timeZone?: TimeZone | s
     const d = new Date(dateObj.getTime());
 
     // Handle UTC calculation
-    if (isUTC(timeZone)) {
+    if (isUTC(zoneName)) {
       d.setUTCMonth(d.getUTCMonth() + months);
       // Adjust to last day of month if new month has fewer days
       if (d.getUTCDate() < dateObj.getUTCDate()) {

tests/addDays.spec.ts

@@ -112,7 +112,14 @@ describe('Local Time', () => {
 });
 
 describe('UTC Time', () => {
-  beforeAll(() => (process.env.TZ = 'Asia/Tokyo'));
+  beforeAll(() => (process.env.TZ = 'America/Los_Angeles'));
+
+  test('UTC timezone object across the daylight-saving transition', () => {
+    const date1 = new Date(Date.UTC(2024, 2, 9, 12));
+    const date2 = new Date(Date.UTC(2024, 2, 10, 12));
+
+    expect(addDays(date1, 1, { zone_name: 'UTC', gmt_offset: [0] })).toEqual(date2);
+  });
 
   test('One day after 2:00 AM on January 31, 2024', () => {
     const date1 = new Date(Date.UTC(2024, 0, 31, 10));

tests/addMonths.spec.ts

@@ -112,7 +112,14 @@ describe('Local Time', () => {
 });
 
 describe('UTC Time', () => {
-  beforeAll(() => (process.env.TZ = 'Asia/Tokyo'));
+  beforeAll(() => (process.env.TZ = 'America/Los_Angeles'));
+
+  test('UTC timezone object at the end of January', () => {
+    const date1 = new Date(Date.UTC(2025, 0, 31, 0, 30));
+    const date2 = new Date(Date.UTC(2025, 1, 28, 0, 30));
+
+    expect(addMonths(date1, 1, { zone_name: 'UTC', gmt_offset: [0] })).toEqual(date2);
+  });
 
   test('One month after 2:00 AM on January 31, 2024', () => {
     const date1 = new Date(Date.UTC(2024, 0, 31, 10));

tests/addYears.spec.ts

@@ -112,7 +112,14 @@ describe('Local Time', () => {
 });
 
 describe('UTC Time', () => {
-  beforeAll(() => (process.env.TZ = 'Asia/Tokyo'));
+  beforeAll(() => (process.env.TZ = 'America/Los_Angeles'));
+
+  test('UTC timezone object when clamping a leap day', () => {
+    const date1 = new Date(Date.UTC(2024, 1, 29, 0, 30));
+    const date2 = new Date(Date.UTC(2025, 1, 28, 0, 30));
+
+    expect(addYears(date1, 1, { zone_name: 'UTC', gmt_offset: [0] })).toEqual(date2);
+  });
 
   test('One year after 2:00 AM on January 31, 2024', () => {
     const date1 = new Date(Date.UTC(2024, 0, 31, 10));