fix: Retain UTC mode when constructing a new instance from another in…

…stance in UTC mode

src/index.js

@@ -29,7 +29,11 @@ const parseLocale = (preset, object, isLocal) => {
 
 const dayjs = (date, c) => {
   if (isDayjs(date)) {
-    return date.clone()
+    if (c) {
+      date = date.toDate()
+    } else {
+      return date.clone()
+    }
   }
   const cfg = c || {}
   cfg.date = date

test/parse.test.js

@@ -79,6 +79,22 @@ it('Does not apply the UTC mode by default', () => {
   expect(instance.minute()).toEqual(34)
 })
 
+it('Creates an UTC instance from another instance', () => {
+  const source = dayjs('2018-09-06')
+  const instance = dayjs(source, { utc: true })
+  expect(instance.$u).toBeTruthy()
+  expect(instance.hour()).toEqual(source.toDate().getUTCHours())
+  expect(instance.minute()).toEqual(source.toDate().getUTCMinutes())
+})
+
+it('Creating a new instance from another instance retains the UTC mode', () => {
+  const source = dayjs('2018-09-06', { utc: true })
+  const instance = dayjs(source)
+  expect(instance.$u).toBeTruthy()
+  expect(instance.hour()).toEqual(source.hour())
+  expect(instance.minute()).toEqual(source.minute())
+})
+
 it('Clone not affect each other', () => {
   const base = dayjs(20170101)
   const year = base.year()
@@ -93,3 +109,9 @@ it('Clone with same value', () => {
   const another = newBase.clone()
   expect(newBase.toString()).toBe(another.toString())
 })
+
+it('Clone retains the UTC mode', () => {
+  const instance = dayjs('2018-09-06', { utc: true })
+  const another = instance.clone()
+  expect(another.$u).toBeTruthy()
+})