Add some tests for 'YYYY' format

test/display.test.js

@@ -260,3 +260,15 @@ it('As JSON -> toJSON', () => {
 it('As ISO 8601 String -> toISOString e.g. 2013-02-04T22:44:30.652Z', () => {
   expect(dayjs().toISOString()).toBe(moment().toISOString())
 })
+
+it('Year 1 formatted with YYYY should pad with zeroes', () => {
+  const date = new Date(1, 0, 1)
+  date.setUTCFullYear(1) // Required because 0-99 are parsed as 19xx in JS: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/Date#year
+  expect(dayjs(date).format('YYYY')).toBe('0001')
+})
+
+it('Year 1 formatting matches moment format', () => {
+  const date = new Date(1, 0, 1)
+  date.setUTCFullYear(1)
+  expect(dayjs(date).format('YYYY')).toBe(moment(date).format('YYYY'))
+})