Java Convert GMT/UTC to Local time doesn't work as expected

In Order to show a reproducible scenario, I am doing the following

1. Get the current system time (local time)

2. Convert Local time to UTC // Works Fine Till here

3. Reverse the UTC time, back to local time. Followed 3 different approaches (listed below) but all the 3 approaches retains the time in UTC only.

   {

   long ts = System.currentTimeMillis();
   Date localTime = new Date(ts);
   String format = "yyyy/MM/dd HH:mm:ss";
   SimpleDateFormat sdf = new SimpleDateFormat (format);
   
   // Convert Local Time to UTC (Works Fine) 
   sdf.setTimeZone(TimeZone.getTimeZone("UTC"));
   Date gmtTime = new Date(sdf.format(localTime));
   System.out.println("Local:" + localTime.toString() + "," + localTime.getTime() + " --> UTC time:" + gmtTime.toString() + "-" + gmtTime.getTime());
   
   // Reverse Convert UTC Time to Locale time (Doesn't work) Approach 1
   sdf.setTimeZone(TimeZone.getDefault());        
   localTime = new Date(sdf.format(gmtTime));
   System.out.println("Local:" + localTime.toString() + "," + localTime.getTime() + " --> UTC time:" + gmtTime.toString() + "-" + gmtTime.getTime());
   
   // Reverse Convert UTC Time to Locale time (Doesn't work) Approach 2 using DateFormat
   DateFormat df = new SimpleDateFormat (format);
   df.setTimeZone(TimeZone.getDefault());
   localTime = df.parse((df.format(gmtTime)));
   System.out.println("Local:" + localTime.toString() + "," + localTime.getTime() + " --> UTC time:" + gmtTime.toString() + "-" + gmtTime.getTime());
   
   // Approach 3
   Calendar c = new GregorianCalendar(TimeZone.getDefault());
   c.setTimeInMillis(gmtTime.getTime());
   System.out.println("Local Time " + c.toString());
   

   }

Solution 1:

I also recommend using Joda as mentioned before.

Solving your problem using standard Java Date objects only can be done as follows:

    // **** YOUR CODE **** BEGIN ****
    long ts = System.currentTimeMillis();
    Date localTime = new Date(ts);
    String format = "yyyy/MM/dd HH:mm:ss";
    SimpleDateFormat sdf = new SimpleDateFormat(format);

    // Convert Local Time to UTC (Works Fine)
    sdf.setTimeZone(TimeZone.getTimeZone("UTC"));
    Date gmtTime = new Date(sdf.format(localTime));
    System.out.println("Local:" + localTime.toString() + "," + localTime.getTime() + " --> UTC time:"
            + gmtTime.toString() + "," + gmtTime.getTime());

    // **** YOUR CODE **** END ****

    // Convert UTC to Local Time
    Date fromGmt = new Date(gmtTime.getTime() + TimeZone.getDefault().getOffset(localTime.getTime()));
    System.out.println("UTC time:" + gmtTime.toString() + "," + gmtTime.getTime() + " --> Local:"
            + fromGmt.toString() + "-" + fromGmt.getTime());

Output:

Local:Tue Oct 15 12:19:40 CEST 2013,1381832380522 --> UTC time:Tue Oct 15 10:19:40 CEST 2013,1381825180000
UTC time:Tue Oct 15 10:19:40 CEST 2013,1381825180000 --> Local:Tue Oct 15 12:19:40 CEST 2013-1381832380000

Solution 2:

You have a date with a known timezone (Here Europe/Madrid), and a target timezone (UTC)

You just need two SimpleDateFormats:

        long ts = System.currentTimeMillis();
        Date localTime = new Date(ts);

        SimpleDateFormat sdfLocal = new SimpleDateFormat ("yyyy/MM/dd HH:mm:ss");
        sdfLocal.setTimeZone(TimeZone.getTimeZone("Europe/Madrid"));

        SimpleDateFormat sdfUTC = new SimpleDateFormat ("yyyy/MM/dd HH:mm:ss");
        sdfUTC.setTimeZone(TimeZone.getTimeZone("UTC"));

        // Convert Local Time to UTC
        Date utcTime = sdfLocal.parse(sdfUTC.format(localTime));
        System.out.println("Local:" + localTime.toString() + "," + localTime.getTime() + " --> UTC time:" + utcTime.toString() + "-" + utcTime.getTime());

        // Reverse Convert UTC Time to Locale time
        localTime = sdfUTC.parse(sdfLocal.format(utcTime));
        System.out.println("UTC:" + utcTime.toString() + "," + utcTime.getTime() + " --> Local time:" + localTime.toString() + "-" + localTime.getTime());

So after see it working you can add this method to your utils:

    public Date convertDate(Date dateFrom, String fromTimeZone, String toTimeZone) throws ParseException {
        String pattern = "yyyy/MM/dd HH:mm:ss";
        SimpleDateFormat sdfFrom = new SimpleDateFormat (pattern);
        sdfFrom.setTimeZone(TimeZone.getTimeZone(fromTimeZone));

        SimpleDateFormat sdfTo = new SimpleDateFormat (pattern);
        sdfTo.setTimeZone(TimeZone.getTimeZone(toTimeZone));

        Date dateTo = sdfFrom.parse(sdfTo.format(dateFrom));
        return dateTo;
    }