Fix age calc, missing leap day for years divisible by 100 and not by 400 (#96)

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);

src/util/age_util.ts

@@ -115,10 +115,21 @@ function calcDateDifferenceInYears(
   const firstDateObject = toDateObject(firstDate);
   const secondDateObject = toDateObject(secondDate);
 
-  const dateDiff = new Date(
+  const startYear = firstDateObject.getUTCFullYear();
-    secondDateObject.valueOf() - firstDateObject.valueOf(),
+
-  );
+  let yearDiff = secondDateObject.getUTCFullYear() - startYear;
-  return Math.abs(dateDiff.getUTCFullYear() - 1970);
+  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(