From 35259e57674fda869078f4da9d8cc37f10b6a956 Mon Sep 17 00:00:00 2001
From: czifumasa <lukasz.lenart912@gmail.com>
Date: Wed, 27 Apr 2022 17:16:23 +0200
Subject: [PATCH] Fix age calc, missing leap day for years divisible by 100 and
 not by 400 (#96)

---
 src/util/age_util.spec.ts | 12 ++++++++++++
 src/util/age_util.ts      | 19 +++++++++++++++----
 2 files changed, 27 insertions(+), 4 deletions(-)

diff --git a/src/util/age_util.spec.ts b/src/util/age_util.spec.ts
index 072f7c9..1058d1d 100644
--- a/src/util/age_util.spec.ts
+++ b/src/util/age_util.spec.ts
@@ -32,6 +32,18 @@ describe('calcAge()', () => {
     const age = calcAge('2 Sep 1990', '1 Sep 2021', intl);
     expect(age).toEqual('30 years');
   });
+  it('age respecting missing leap year divisible by 100 and not divisible by 400', () => {
+    const age = calcAge('1890', '1921', intl);
+    expect(age).toEqual('31 years');
+  });
+  it('age respecting missing leap year divisible by 100 and not divisible by 400 for full dates', () => {
+    const age = calcAge('1 Sep 1890', '1 Sep 1921', intl);
+    expect(age).toEqual('31 years');
+  });
+  it('age with round down respecting missing leap year divisible by 100 and not divisible by 400', () => {
+    const age = calcAge('2 Sep 1890', '1 Sep 1921', intl);
+    expect(age).toEqual('30 years');
+  });
 
   it('age with exact and range between', () => {
     const age = calcAge('1990', 'BET 2020 AND 2021', intl);
diff --git a/src/util/age_util.ts b/src/util/age_util.ts
index f33e9b7..e307071 100644
--- a/src/util/age_util.ts
+++ b/src/util/age_util.ts
@@ -115,10 +115,21 @@ function calcDateDifferenceInYears(
   const firstDateObject = toDateObject(firstDate);
   const secondDateObject = toDateObject(secondDate);
 
-  const dateDiff = new Date(
-    secondDateObject.valueOf() - firstDateObject.valueOf(),
-  );
-  return Math.abs(dateDiff.getUTCFullYear() - 1970);
+  const startYear = firstDateObject.getUTCFullYear();
+
+  let yearDiff = secondDateObject.getUTCFullYear() - startYear;
+  let monthDiff = secondDateObject.getUTCMonth() - firstDateObject.getUTCMonth();
+  if (monthDiff < 0) {
+    yearDiff--;
+    monthDiff += 12;
+  }
+  const dayDiff = secondDateObject.getUTCDate() - firstDateObject.getUTCDate();
+  if (dayDiff < 0) {
+    if (monthDiff <= 0) {
+      yearDiff--;
+    }
+  }
+  return Math.abs(yearDiff);
 }
 
 export function calcAge(