diff --git a/src/plugin/timezone/index.js b/src/plugin/timezone/index.js
--- a/src/plugin/timezone/index.js
+++ b/src/plugin/timezone/index.js
@@ -59,7 +59,12 @@ export default (o, c, d) => {
   }
   d.tz = function (input, timezone) {
     const previousOffset = tzOffset(+d(), timezone)
-    const localTs = d.utc(input).valueOf()
+    let localTs
+    if (typeof input !== 'string') {
+      // timestamp number || js Date || Day.js
+      localTs = d(input) + (previousOffset * 60 * 1000)
+    }
+    localTs = localTs || d.utc(input).valueOf()
     const [targetTs, targetOffset] = fixOffset(localTs, previousOffset, timezone)
     const ins = d(targetTs).utcOffset(targetOffset)
     return ins
diff --git a/types/plugin/timezone.d.ts b/types/plugin/timezone.d.ts
--- a/types/plugin/timezone.d.ts
+++ b/types/plugin/timezone.d.ts
@@ -1,16 +1,16 @@
-import { PluginFunc } from 'dayjs'
+import { PluginFunc, ConfigType } from 'dayjs'
 
 declare const plugin: PluginFunc
 export = plugin
 
-interface DayjsTimezone {
-  (): Dayjs
-  guess(): string
-}
-
 declare module 'dayjs' {
   interface Dayjs {
-    tz(): Dayjs
+    tz(timezone: string): Dayjs
+  }
+
+  interface DayjsTimezone {
+    (date: ConfigType, timezone: string): Dayjs
+    guess(): string
   }
 
   const tz: DayjsTimezone

diff --git a/test/plugin/timezone.test.js b/test/plugin/timezone.test.js
--- a/test/plugin/timezone.test.js
+++ b/test/plugin/timezone.test.js
@@ -16,6 +16,8 @@ afterEach(() => {
 })
 
 const NY = 'America/New_York'
+const VAN = 'America/Vancouver'
+const TOKYO = 'Asia/Tokyo'
 
 describe('Guess', () => {
   it('return string', () => {
@@ -36,6 +38,19 @@ describe('Parse', () => {
     expect(newYork.valueOf()).toBe(MnewYork.valueOf())
   })
 
+  it('parse timestamp, js Date, Day.js object', () => {
+    const d = new Date('2020-08-07T12:00-07:00')
+    const result = '2020-08-07T12:00:00-07:00'
+    const TjsDate = dayjs.tz(d, VAN)
+    const Tdayjs = dayjs.tz(dayjs(d), VAN)
+    const Timestamp = dayjs.tz(d.getTime(), VAN)
+    const Tmoment = moment.tz(d, VAN)
+    expect(TjsDate.format()).toBe(result)
+    expect(Tdayjs.format()).toBe(result)
+    expect(Timestamp.format()).toBe(result)
+    expect(Tmoment.format()).toBe(result)
+  })
+
   it('parse and convert between timezones', () => {
     const newYork = dayjs.tz('2014-06-01 12:00', NY)
     expect(newYork.tz('America/Los_Angeles').format()).toBe('2014-06-01T09:00:00-07:00')
@@ -71,8 +86,8 @@ describe('Convert', () => {
       expect(dec.tz('America/Los_Angeles').format('ha')).toBe('4am')
       expect(jun.tz(NY).format('ha')).toBe('8am')
       expect(dec.tz(NY).format('ha')).toBe('7am')
-      expect(jun.tz('Asia/Tokyo').format('ha')).toBe('9pm')
-      expect(dec.tz('Asia/Tokyo').format('ha')).toBe('9pm')
+      expect(jun.tz(TOKYO).format('ha')).toBe('9pm')
+      expect(dec.tz(TOKYO).format('ha')).toBe('9pm')
       expect(jun.tz('Australia/Sydney').format('ha')).toBe('10pm')
       expect(dec.tz('Australia/Sydney').format('ha')).toBe('11pm')
     })
@@ -80,8 +95,8 @@ describe('Convert', () => {
 
   it('format Z', () => {
     [dayjs, moment].forEach((_) => {
-      const losAngeles = _('2020-08-06T03:48:10.258Z').tz('Asia/Tokyo')
-      expect(losAngeles.format('Z')).toBe('+09:00')
+      const t = _('2020-08-06T03:48:10.258Z').tz(TOKYO)
+      expect(t.format('Z')).toBe('+09:00')
     })
   })
 })

TypeScript error when using Timezone plugin: `timezone.d.ts(7,7): error TS2304: Cannot find name 'Dayjs'`
I got this error: `timezone.d.ts(7,7): error TS2304: Cannot find name 'Dayjs'` when I use `dayjs.extend(timezone);`

I think that `DayjsTimezone` interface should be declared inside the module declaration
https://github.com/iamkun/dayjs/blob/e69caba1b0957241a855aa0ae38db899fa2c3795/types/plugin/timezone.d.ts#L6-L17

or `Dayjs` import is missing here
https://github.com/iamkun/dayjs/blob/e69caba1b0957241a855aa0ae38db899fa2c3795/types/plugin/timezone.d.ts#L1

Yes, it's definitely a bug.
test/timezone.test.js(1) test/timezone.test.js(2) test/timezone.test.js(3) test/timezone.test.js(4)
test/plugin/timezone.test.js