fix(date): keep birthdate within the age range when refDate is a leap day

src/modules/date/birthdate.ts

@@ -4,6 +4,24 @@ import { toDate } from '../../internal/date';
 import { getDefaultRefDate } from '../../utils/get-default-ref-date';
 import { fakeDateBetween } from './between';
 
+/**
+ * Returns the timestamp of the given date moved to the given year.
+ * February 29 becomes February 28 in a common year instead of rolling over to March 1,
+ * so a person born on the returned day has their birthday on or before the given date.
+ *
+ * @param date The date to move.
+ * @param year The year to move the date to.
+ */
+function sameDayInYear(date: Date, year: number): number {
+  const result = new Date(date);
+  result.setUTCFullYear(year);
+  if (result.getUTCMonth() !== date.getUTCMonth()) {
+    result.setUTCDate(0);
+  }
+
+  return result.getTime();
+}
+
 /**
  * Returns a random birthdate. By default, the birthdate is generated for an adult between 18 and 80 years old.
  * But you can customize the `'age'` range or the `'year'` range to generate a more specific birthdate.
@@ -184,8 +202,8 @@ export function fakeDateBirthdate(
     case 'age': {
       // Add one day to the `from` date to avoid generating the same date as the reference date.
       const oneDay = 24 * 60 * 60 * 1000;
-      const from = new Date(refDate).setUTCFullYear(refYear - max - 1) + oneDay;
-      const to = new Date(refDate).setUTCFullYear(refYear - min);
+      const from = sameDayInYear(refDate, refYear - max - 1) + oneDay;
+      const to = sameDayInYear(refDate, refYear - min);
 
       if (from > to) {
         throw new FakerError(

test/modules/date.spec.ts

@@ -952,6 +952,50 @@ describe('date', () => {
           expect(age).toBeLessThanOrEqual(22);
         });
 
+        it('returns a birthdate of the min age when the refDate is a leap day', () => {
+          const customFaker = new SimpleFaker({
+            randomizer: {
+              next() {
+                return 0.999999999999999;
+              },
+              seed() {},
+            },
+          });
+          const refDate = new Date(Date.UTC(2024, 1, 29, 12));
+
+          const birthdate = customFaker.date.birthdate({
+            min: 18,
+            max: 18,
+            refDate,
+            mode: 'age',
+          });
+
+          // Born on 2006-03-01, the person turns 18 the day after the refDate.
+          expect(birthdate).toEqual(new Date(Date.UTC(2006, 1, 28, 12)));
+        });
+
+        it('returns a birthdate of the max age when the refDate is a leap day', () => {
+          const customFaker = new SimpleFaker({
+            randomizer: {
+              next() {
+                return 0;
+              },
+              seed() {},
+            },
+          });
+          const refDate = new Date(Date.UTC(2024, 1, 29, 12));
+
+          const birthdate = customFaker.date.birthdate({
+            min: 80,
+            max: 80,
+            refDate,
+            mode: 'age',
+          });
+
+          // Born on 1943-03-01, the person is still 80 on the refDate.
+          expect(birthdate).toEqual(new Date(Date.UTC(1943, 2, 1, 12)));
+        });
+
         it('should throw an error when the min > max year', () => {
           const min = 2000;
           const max = 1990;