Handle dates in year 99 rolling over into year 100 behaving as if yea…

…r 100 was a leap year (#1390)

Fix #1389

src/impl/util.js

@@ -166,7 +166,10 @@ export function objToLocalTS(obj) {
   // for legacy reasons, years between 0 and 99 are interpreted as 19XX; revert that
   if (obj.year < 100 && obj.year >= 0) {
     d = new Date(d);
-    d.setUTCFullYear(d.getUTCFullYear() - 1900);
+    // set the month and day again, this is necessary because year 2000 is a leap year, but year 100 is not
+    // so if obj.year is in 99, but obj.day makes it roll over into year 100,
+    // the calculations done by Date.UTC are using year 2000 - which is incorrect
+    d.setUTCFullYear(obj.year, obj.month - 1, obj.day);
   }
   return +d;
 }

test/datetime/math.test.js

@@ -107,6 +107,13 @@ test("DateTime#plus works across the 100 barrier", () => {
   expect(d.day).toBe(2);
 });
 
+test("DateTime#plus works across the 100 barrier when passing through February", () => {
+  const d = DateTime.fromISO("0099-12-31").plus({ day: 61 });
+  expect(d.year).toBe(100);
+  expect(d.month).toBe(3);
+  expect(d.day).toBe(2);
+});
+
 test("DateTime#plus renders invalid when out of max. datetime range using days", () => {
   const d = DateTime.utc(1970, 1, 1, 0, 0, 0, 0).plus({ day: 1e8 + 1 });
   expect(d.isValid).toBe(false);