For Question # 1: The timezone offset can be the same for different timezones, but DST can be used or not, and this leads to a difference.

For question number 2:

Going forward, you can only be safe while using UTC. (you can work if your time data is "latest" - see below)

In the past, you cannot reliably extract the correct time.

General advice for conversions:

I was working on a project related to time zones and DST in a JDBC driver. There were problems with storing time values ​​and correcting them correctly. I was working / really hard / trying to get the right to convert, so we could save the larger work of switching to UTC. There is no correct conversion without UTC. (/ real hard /: Think of Pulp Fiction, where Jules says, "I try really hard to be a shepherd. :-))

Question # 2 / Future:

If your client is unable to send UTC time (possibly because it is a third party system):

When your server receives time data (not UTC) from a client that you know is current for a few minutes (maybe a little longer), you can try using UTC time and match that to the client. Imagine that your client sends "2015-06-01 15:45" and you know it is "2015-06-01 18:51 UTC", then you can interpret the client's time as "2015-06-01 18: 45 UNIVERSAL GLOBAL TIME ". If the time data sent by the customer may be older than an hour, in some cases this will fail.

Or in other words: Tell your customer to record temperature values. If the data sent by the client is no older than a few minutes, you can map that to UTC time. If your client records the temperature on the same day and sends it to you at the end of the day, you can't match the time correctly.

Why can't you convert completely (!)?

Suppose the night that daylight saving time changes is changing so that the hours change from 03:00 to 02:00. You have 02:30 before the switch and another 02:30 after the switch. The first 02:30 has a different UTC time than the second 02:30. So you're fine with UTC. But only with "client local" 02:30 you will never be sure.

Back to client data age: If your client sends data no older than a few minutes at 02:30 and then a second at the second 02:30, you can highlight this on the server. If at 04:00 you receive two entries in 02:30, you will no longer be able to recover UTC.

Question # 2 / Past:

Can you add a flag to the database so that new times that were passed as UTC are marked "reliable" and the old values ​​are not?

Output and source:

Result of working with a modified source on my system, which has a TZ "Europe / Berlin". Note that this uses DST, but the first TZ received ("Algeria") does not use DST.

formatter TZ is sun.util.calendar.ZoneInfo[id="Europe/Berlin",offset=3600000,dstSavings=3600000,useDaylight=true,transitions=143,lastRule=java.util.SimpleTimeZone[id=Europe/Berlin,offset=3600000,dstSavings=3600000,useDaylight=true,startYear=0,startMode=2,startMonth=2,startDay=-1,startDayOfWeek=1,startTime=3600000,startTimeMode=2,endMode=2,endMonth=9,endDay=-1,endDayOfWeek=1,endTime=3600000,endTimeMode=2]]
internal date value = 1433109600000 as UTC = 31/05/2015::10:00:00 PM
Before conversion --> 01/06/2015::12:00:00 AM
Conversion: offset != 0, using TZ sun.util.calendar.ZoneInfo[id="Africa/Algiers",offset=3600000,dstSavings=0,useDaylight=false,transitions=35,lastRule=null]
After conversion --> 31/05/2015::11:00:00 PM

Setting UTC...

formatter TZ is sun.util.calendar.ZoneInfo[id="UTC",offset=0,dstSavings=0,useDaylight=false,transitions=0,lastRule=null]
internal date value = 1433116800000 as UTC = 01/06/2015::12:00:00 AM
Before conversion --> 01/06/2015::12:00:00 AM
Conversion: offset != 0, using TZ sun.util.calendar.ZoneInfo[id="Africa/Algiers",offset=3600000,dstSavings=0,useDaylight=false,transitions=35,lastRule=null]
After conversion --> 01/06/2015::01:00:00 AM

      

        
        
        
      

    

Source:

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.TimeZone;

public class TimeTest {
    static TimeZone utc = TimeZone.getTimeZone("UTC");

    public static void main (String args[]) {

        SimpleDateFormat formatter = new SimpleDateFormat("dd/MM/yyyy::hh:mm:ss a");
        String dateInString = "01/06/2015::12:00:00 AM";
        SimpleDateFormat utcformatter = new SimpleDateFormat("dd/MM/yyyy::hh:mm:ss a");
        utcformatter.setTimeZone(utc);

        try {    
            Date date = formatter.parse(dateInString);
            System.out.println("formatter TZ is " + formatter.getTimeZone());
            System.out.println("internal date value = " +  date.getTime() + " as UTC = " + utcformatter.format(date));
            System.out.println("Before conversion --> " + formatter.format(date));
            System.out.println("After conversion --> " + convertDateValueIntoString(date));

            System.out.println("\nSetting UTC...\n");
            formatter.setTimeZone(utc);

            date = formatter.parse(dateInString);
            System.out.println("formatter TZ is " + formatter.getTimeZone());
            System.out.println("internal date value = " +  date.getTime() + " as UTC = " + utcformatter.format(date));
            System.out.println("Before conversion --> " + formatter.format(date));
            System.out.println("After conversion --> " + convertDateValueIntoString(date));
        } catch (ParseException e) {
            e.printStackTrace();
        }       
    }

    private static String convertDateValueIntoString(Date dateValue){
        SimpleDateFormat dateFormat = new SimpleDateFormat("dd/MM/yyyy::hh:mm:ss a");       
        String date;
        int offset = TimeZone.getDefault().getRawOffset();
        if (offset == 0) {
            System.out.println("Conversion: offset == 0 -- setting UTC");
            dateFormat.setTimeZone(TimeZone.getTimeZone("UTC"));
            date = dateFormat.format(dateValue);
        } else {        
            String TZ[] = TimeZone.getAvailableIDs(offset);
            String timeZone = TZ[0];
            if (timeZone == null) {
                System.out.println("Conversion: offset != 0, did not find TZ, tz of dateFormat is " + dateFormat.getTimeZone());
                date = dateFormat.format(dateValue);
            } else {
                TimeZone tz = TimeZone.getTimeZone(timeZone);
                System.out.println("Conversion: offset != 0, using TZ " + tz);
                dateFormat.setTimeZone(tz);
                date = dateFormat.format(dateValue);
            }           
        }

        return date;
    }
}