Incorrect behavior of dayjs.tz

[salmannotkhan]

Describe the bug

const date = new Date() // utc timezone date
// 2024-09-04T10:04:12.774Z

dayjs.tz(date, "Asia/Kolkata").toISOString()
// 2024-09-04T10:04:12.774Z

dayjs.tz(date.toISOString(), "Asia/Kolkata").toISOString()
// 2024-09-04T04:34:12.774Z

Expected behavior

dayjs.tz(date, "Asia/Kolkata").toISOString()
// 2024-09-04T04:34:12.774Z

Information

• Day.js Version: v1.11.13
• OS: macOS
• Browser: Node.js
• Time zone: UTC

[salmannotkhan]

I have tracked down the issue. It's here:

dayjs/src/plugin/timezone/index.js

Lines 139 to 142 in 365f81d

	if (typeof input !== 'string') {
	// timestamp number || js Date || Day.js
	return d(input).tz(timezone)
	}

It should be

if (typeof input !== 'string') { 
   if (input instanceof Date || input instanceof Day.js) {
      input = input.toISOString() 
   } else {
      // timestamp number
      input = new Date(input).toISOString() // or dayjs(input).toISOString()
   }
 }