Conversion From PDT TimeZone to IST Timezone is Incorrect - ios

Question

I had set time to San Carlos California(PDT Timezone) in my I phone, then I am trying to add 30 days which is fine , but when I am trying to convert that Date object to IST Timezone , it is not correct.
11/19/2020 06:46:25
Above one is days added by 30 in todays date, In San Calos(PDT Timezone)
when I am converting that time in IST it is "11/18/2020 at 11:46 PM"
but it should be "11/18/2020 at 19:46"
time is variant..
I am using this to convert time into IST
Convert date to local TimeZone
func convertExpiryToIst(inputFormat: String, outputFormat: String) -> String{
let formatter = DateFormatter()
formatter.dateFormat = inputFormat
formatter.timeZone = TimeZone.current
var date = formatter.date(from: self)
date?.addTimeInterval(TimeInterval(value))
formatter.dateFormat = outputFormat
if date != nil {
print(date)
return formatter.string(from: date!)
}
return self
}

Answer 1

The issue there is that you are adding a time interval to the resulting date. A date has no timezone it is just a point in time. All you need to do is to change the date formatter timezone to the desired one and get the string representation of it.
extension String {
func convertExpiryTo(inputFormat: String, inputTimeZone: String, outputFormat: String, outputTimeZone: String)-> String? {
let formatter = DateFormatter()
formatter.dateFormat = inputFormat
formatter.timeZone = TimeZone(identifier: inputTimeZone)
if let date = formatter.date(from: self) {
// let offset = TimeZone(abbreviation: "IST")!.secondsFromGMT(for: date) - TimeZone(abbreviation: "PDT")!.secondsFromGMT(for: date)
// let hours = Double(offset) / 3600 // 13.5 hours
// dont add the timezone offset
// date?.addTimeInterval(TimeInterval(value))
formatter.dateFormat = outputFormat
// just set a different timezone
formatter.timeZone = TimeZone(identifier: outputTimeZone)
return formatter.string(from: date)
}
return nil
}
}
"11/19/2020 06:46:25".convertExpiry(inputFormat: "MM/dd/yyyy HH:mm:ss", inputTimeZone: "America/Los_Angeles", outputFormat: "MM/dd/yyyy 'at' h:mm a", outputTimeZone: "Asia/Kolkata") // "11/19/2020 at 8:16 PM" = +13.5 hours