convert one time zone to another using joda time

Question

there is a form having a drop down for country ,user will select the country ,then there is a drop down for time zone ,user will select the time zone which are available in country selected by the user.Then user will enter the local date( eg: 26-Dec-2014) and time( 23:11)(24 hours time) this entered date and time is for the selected country and time zone. now i have to convert this date and time to GMT time zone. how can i do this using joda time

how the daylight saving time(DST) will be calculate?

i have made a function which accepts the parameters as from time zone,to time zone,date

public static String convertTimeZones( String fromTimeZoneString, 
             String toTimeZoneString, String fromDateTime) {
         DateTimeZone fromTimeZone = DateTimeZone.forID(fromTimeZoneString);
         DateTimeZone toTimeZone = DateTimeZone.forID(toTimeZoneString);
         DateTime dateTime = new DateTime(fromDateTime, fromTimeZone);

         DateTimeFormatter outputFormatter 
            = DateTimeFormat.forPattern("dd-MMM-yyyy HH:mm").withZone(toTimeZone);
        return outputFormatter.print(dateTime);
    }

i want to pass the date to this function in a format (24-Feb-2014 12:34) but it is not taking this format

Answer 1

//so we can get the local date
//UTC = true, translate from UTC time to local
//UTC = false, translate from local to UTC

public static String formatUTCToLocalAndBackTime(String datetime, boolean UTC) { 
    String returnTimeDate = "";
    DateTime dtUTC = null;
    DateTimeZone timezone = DateTimeZone.getDefault();
    DateTimeFormatter formatDT = DateTimeFormat.forPattern("yyyy-MM-dd HH:mm:ss");
    DateTime dateDateTime1 = formatDT.parseDateTime(datetime);
    DateTime now = new DateTime();
    DateTime nowUTC = new LocalDateTime(now).toDateTime(DateTimeZone.UTC);
    long instant = now.getMillis();
    long instantUTC = nowUTC.getMillis();
    long offset = instantUTC - instant;

    if (UTC) { 
        //convert to local time
        dtUTC = dateDateTime1.withZoneRetainFields(DateTimeZone.UTC);
        //dtUTC = dateDateTime1.toDateTime(timezone);
        dtUTC = dtUTC.plusMillis((int) offset);
    } else { 
        //convert to UTC time
        dtUTC = dateDateTime1.withZoneRetainFields(timezone);
        dtUTC = dtUTC.minusMillis((int) offset);        
    }

    returnTimeDate = dtUTC.toString(formatDT);



    return returnTimeDate;
}

Answer 2

Java time api is better compared to joda time api when it comes to various timezones, specially with day light savings

import java.time.LocalDateTime;
import java.time.ZoneId;
import java.time.ZoneOffset;

   LocalDateTime start = LocalDateTime.of(2021, 02, 25, 14, 25, 00);
   ZoneId zone = ZoneId.of("Asia/Singapore");
   start = start.atZone(zone).withZoneSameInstant(ZoneOffset.UTC).toLocalDateTime();