add

node_modules/dayjs/esm/plugin/isLeapYear/index.d.ts

@@ -0,0 +1,10 @@
+import { PluginFunc } from 'dayjs/esm'
+
+declare const plugin: PluginFunc
+export = plugin
+
+declare module 'dayjs/esm' {
+  interface Dayjs {
+    isLeapYear(): boolean
+  }
+}

node_modules/dayjs/esm/plugin/isLeapYear/index.js

@@ -0,0 +1,7 @@
+export default (function (o, c) {
+  var proto = c.prototype;
+
+  proto.isLeapYear = function () {
+    return this.$y % 4 === 0 && this.$y % 100 !== 0 || this.$y % 400 === 0;
+  };
+});