Unable to convert UTC to IST, it is still returning me UTC only

Question

I am trying to convert UTC Date to IST. But to my surprise, after converting everything, it is still returning me UTC only. How is it possible?

INPUT:

StartDateTimeUtc='2017-09-15T14:00:00', 
EndDateTimeUtc='2017-09-15T15:00:00'

Code:

public static final String DATE_FORMATE_CURRENT = "yyyy-MM-dd'T'HH:mm:ss";

Date meetingStartDate = new SimpleDateFormat(Constants.DATE_FORMATE_CURRENT, Locale.ENGLISH).parse(model.StartDateTimeUtc);
Date meetingEndDate = new SimpleDateFormat(Constants.DATE_FORMATE_CURRENT, Locale.ENGLISH).parse(model.EndDateTimeUtc);

            //Convert Date to String
            DateFormat df = new SimpleDateFormat(Constants.DATE_FORMATE_CURRENT);
            String meetinStartDateString = df.format(meetingStartDate);
            String meetingEndDateString = df.format(meetingEndDate);

            //Convert String Date to IST
            SimpleDateFormat dftwo = new SimpleDateFormat(Constants.DATE_FORMATE_CURRENT);
            dftwo.setTimeZone(TimeZone.getTimeZone("Asia/Kolkata"));
            Date date = null;
            Date datetwo = null;
            try {
                date = dftwo.parse(meetinStartDateString);
                datetwo = dftwo.parse(meetingEndDateString);
            } catch (ParseException e) {
                e.printStackTrace();
            }

            dftwo.setTimeZone(TimeZone.getDefault());
            String formattedStartDate = dftwo.format(date);
            String formattedEndDate = dftwo.format(datetwo);

            //Convert String Date back to Date format so that we can pass into Calendar code
            Date meetingStartDateFinal = new SimpleDateFormat(Constants.DATE_FORMATE_CURRENT, Locale.ENGLISH).parse(formattedStartDate);
            Date meetingEndDateFinal = new SimpleDateFormat(Constants.DATE_FORMATE_CURRENT, Locale.ENGLISH).parse(formattedEndDate);

OUTPUT again in UTC:

Start Date : Fri Sep 15 14:00:00 GMT+05:30 2017
End Date : Fri Sep 15 15:00:00 GMT+05:30 2017

Answer 1

A java.util.Date doesn't have any timezone information. It just contains one value: the number of milliseconds since unix epoch (1970-01-01T00:00Z, or January 1^st 1907, at midnight in UTC).

This number of milliseconds is the same, everywhere in the world. What's is different is the corresponding date and time in each timezone. Example: right now, this millis value is 1505481835424, which corresponds, in UTC, to 2017-09-15T13:23:55.424Z. This same value corresponds to 10:23 AM in São Paulo, 18:53 in Kolkata, 14:23 in London and so on. The local date/time is different in each timezone, but the millis value is the same for everyone.

That's why you don't convert a Date itself: the millis value is the same, and there's no need to change it. What you can change is the representation of this date in different timezones.

SimpleDateFormat, by default, uses the JVM default timezone to parse dates. But if you know that the inputs are in a specific zone, you must set in the formatter. So, to parse your inputs, you must do:

String startDateTimeUtc = "2017-09-15T14:00:00";
String endDateTimeUtc = "2017-09-15T15:00:00";

SimpleDateFormat sdf = new SimpleDateFormat( "yyyy-MM-dd'T'HH:mm:ss");
// input is in UTC
sdf.setTimeZone(TimeZone.getTimeZone("UTC"));

// parse dates
Date meetingStartDate = sdf.parse(startDateTimeUtc);
Date meetingEndDate = sdf.parse(endDateTimeUtc);

The 2 Date objects above will correspond to 14:00 and 15:00 UTC (which is the same as 19:30 and 20:30 in Kolkata timezone).

But if you just print the Date objects directly (using System.out.println, logging, or even checking their values in a debugger), it'll implicity call the toString() method, which uses the JVM default timezone behind the scenes, resulting in the output you're seeing (Fri Sep 15 14:00:00 GMT+05:30 2017).

If you want to print in a specific format, and in a specific timezone, you'll need another formatter:

// another formatter for output
SimpleDateFormat outputFormat= new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss");
// output will be in Asia/Kolkata timezone
outputFormat.setTimeZone(TimeZone.getTimeZone("Asia/Kolkata"));

System.out.println(outputFormat.format(meetingStartDate));
System.out.println(outputFormat.format(meetingEndDate));

The output will be:

2017-09-15T19:30:00
2017-09-15T20:30:00

Which corresponds to the same UTC dates in Kolkata timezone.

Just remember: you don't convert the Date's between timezones (because their millis values are "absolute" - they are the same for everyone in the world). You just change the String representation of those dates (the corresponding date/time in a specific timezone).

---

Java new Date/Time API

The old classes (Date, Calendar and SimpleDateFormat) have lots of problems and design issues, and they're being replaced by the new APIs.

In Android you can use the ThreeTen Backport, a great backport for Java 8's new date/time classes. To make it work, you'll also need the ThreeTenABP (more on how to use it here).

This new API has lots of different date/time types for each situation. In this case, the inputs have date and time, but no timezone information, so first I parse them to a org.threeten.bp.LocalDateTime, using a org.threeten.bp.format.DateTimeFormatter:

// parse the inputs
DateTimeFormatter fmt = DateTimeFormatter.ISO_LOCAL_DATE_TIME;
LocalDateTime startDt = LocalDateTime.parse(startDateTimeUtc, fmt);
LocalDateTime endDt = LocalDateTime.parse(endDateTimeUtc, fmt);

Then I use a org.threeten.bp.ZoneOffset to convert them to UTC, and later a org.threeten.bp.ZoneId to convert this to another timezone. The result will be a org.threeten.bp.ZonedDateTime:

// input is in UTC
ZoneOffset utc = ZoneOffset.UTC;
// convert to Asia/Kolkata
ZoneId zone = ZoneId.of("Asia/Kolkata");
ZonedDateTime start = startDt.atOffset(utc).atZoneSameInstant(zone);
ZonedDateTime end = endDt.atOffset(utc).atZoneSameInstant(zone);

Then I use the same DateTimeFormatter to format the output:

System.out.println(fmt.format(start));
System.out.println(fmt.format(end));

The output is:

2017-09-15T19:30:00
2017-09-15T20:30:00

Note that I don't need to set the timezone in the formatter, because the timezone information is in the objects (they are responsible to do the conversion).