feature: Update addYears function to make it timezone agnostic conversion

src/addYears/index.ts

@@ -1,5 +1,3 @@
-import { addMonths } from "../addMonths/index.js";
-
 /**
  * @name addYears
  * @category Year Helpers
@@ -8,7 +6,8 @@ import { addMonths } from "../addMonths/index.js";
  * @description
  * Add the specified number of years to the given date.
  *
- * @typeParam DateType - The `Date` type, the function operates on. Gets inferred from passed arguments. Allows to use extensions like [`UTCDate`](https://github.com/date-fns/utc).
+ * @typeParam DateType - The `Date` type, the function operates on. Gets inferred from passed arguments.
+ * Allows to use extensions like [`UTCDate`](https://github.com/date-fns/utc).
  *
  * @param date - The date to be changed
  * @param amount - The amount of years to be added.
@@ -24,5 +23,8 @@ export function addYears<DateType extends Date>(
   date: DateType | number | string,
   amount: number,
 ): DateType {
-  return addMonths(date, amount * 12);
+  // Convert date to UTC to perform timezone agnostic calculations
+  const utcDate = new Date(date);
+  utcDate.setUTCMonth(utcDate.getUTCMonth() + amount * 12);
+  return utcDate as DateType;
 }

src/addYears/test.ts

@@ -3,33 +3,33 @@ import { addYears } from "./index.js";
 
 describe("addYears", () => {
   it("adds the given number of years", () => {
-    const result = addYears(new Date(2014, 8 /* Sep */, 1), 5);
-    expect(result).toEqual(new Date(2019, 8 /* Sep */, 1));
+    const result = addYears(new Date(Date.UTC(2014, 8 /* Sep */, 1)), 5);
+    expect(result).toEqual(new Date(Date.UTC(2019, 8 /* Sep */, 1)));
   });
 
   it("accepts a timestamp", () => {
-    const result = addYears(new Date(2014, 8 /* Sep */, 1).getTime(), 12);
-    expect(result).toEqual(new Date(2026, 8 /* Sep */, 1));
+    const result = addYears(Date.UTC(2014, 8 /* Sep */, 1), 12);
+    expect(result).toEqual(new Date(Date.UTC(2026, 8 /* Sep */, 1)));
   });
 
   it("does not mutate the original date", () => {
-    const date = new Date(2014, 8 /* Sep */, 1);
+    const date = new Date(Date.UTC(2014, 8 /* Sep */, 1));
     addYears(date, 12);
-    expect(date).toEqual(new Date(2014, 8 /* Sep */, 1));
+    expect(date).toEqual(new Date(Date.UTC(2014, 8 /* Sep */, 1)));
   });
 
   it("handles the leap years properly", () => {
-    const result = addYears(new Date(2016, 1 /* Feb */, 29), 1);
-    expect(result).toEqual(new Date(2017, 1 /* Feb */, 28));
+    const result = addYears(new Date(Date.UTC(2016, 1 /* Feb */, 29)), 1);
+    expect(result).toEqual(new Date(Date.UTC(2017, 1 /* Feb */, 28)));
   });
 
   it("handles dates before 100 AD", () => {
     const initialDate = new Date(0);
-    initialDate.setFullYear(0, 1 /* Feb */, 29);
-    initialDate.setHours(0, 0, 0, 0);
+    initialDate.setUTCFullYear(0, 1 /* Feb */, 29);
+    initialDate.setUTCHours(0, 0, 0, 0);
     const expectedResult = new Date(0);
-    expectedResult.setFullYear(1, 1 /* Feb */, 28);
-    expectedResult.setHours(0, 0, 0, 0);
+    expectedResult.setUTCFullYear(1, 1 /* Feb */, 28);
+    expectedResult.setUTCHours(0, 0, 0, 0);
     const result = addYears(initialDate, 1);
     expect(result).toEqual(expectedResult);
   });
@@ -40,7 +40,14 @@ describe("addYears", () => {
   });
 
   it("returns `Invalid Date` if the given amount is NaN", () => {
-    const result = addYears(new Date(2014, 8 /* Sep */, 1), NaN);
+    const result = addYears(new Date(Date.UTC(2014, 8 /* Sep */, 1)), NaN);
     expect(result instanceof Date && isNaN(result.getTime())).toBe(true);
   });
+
+  it("operates on UTC dates", () => {
+    const date = new Date(Date.UTC(2014, 8 /* Sep */, 1));
+    const result = addYears(date, 5);
+    const expectedResult = new Date(Date.UTC(2019, 8 /* Sep */, 1));
+    expect(result).toEqual(expectedResult);
+  });
 });