Date

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• JavaScript: Difference between two dates in years, months, days

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Daylight saving time or winter time changeover (times in CET)

Central European Time (CET), used in most parts of Europe and a few North African countries, is a standard time which is 1 hour ahead of Coordinated Universal Time (UTC). The time offset from UTC can be written as UTC+01:00.

European countries using CET in the winter and CEST in the summer:

Currently Central European Summer Time (CEST), UTC +2
Standard time (Central European Time (CET), UTC +1) starts October 25, 2020

CET alternates between UTC+1 (standard time) and UTC+2 (when daylight saving time (DST) is observed). All countries in the CET time zone observe DST (UTC+2) from 02:00 am on the last Sunday of March until 03:00 am on the last Sunday of October.

Time zone : https://time.is/CET

• https://www.worldtimebuddy.com/
• https://www.epochconverter.com/

Lenient

Specifies whether or not date/time interpretation is to be lenient. With lenient interpretation, a date such as "February 942, 1996" will be treated as being equivalent to the 941st day after February 1, 1996. With strict (non-lenient) interpretation, such dates will cause an exception to be thrown. The default is lenient.

dateFormat.setLenient(false);
Parameters:lenient true if the lenient mode is to be turned on; false if it is to be turned off.

CET = UTC+01:00 ShortDay(Gap), LongDay(Overlap)
TransitionRule[Gap +01:00 to  , SUNDAY on or after MARCH 25 at 02:00 STANDARD, standard offset +01:00]
TransitionRule[Overlap +02:00 to +01:00, SUNDAY on or after OCTOBER 25 at 02:00 STANDARD, standard offset +01:00]

Example:

public class ShortDayTest {
    public static void main(String[] args) throws ParseException {
        
        String[] datesArr = {
                "2020-03-29T01:00:00", "2020-03-29T02:00:00", "2020-03-29T03:00:00", "2020-03-29T04:00:00", // Short Day on 2nd hour
                "2020-10-25T01:00:00", "2020-10-25T02:00:00", "2020-10-25T03:00:00"  // Long  Day on 2nd hour
                };
        
        String timeZone = System.getProperty("user.timezone");
        System.out.println("Time Zone:"+timeZone);
        
        //TimeZone.setDefault( TimeZone.getTimeZone("UTC") ); // Default: java.sql.Timestam. Change: GMT, CET, UTC
        //System.setProperty("user.timezone", "UTC"); // or -Duser.timezone=GMT in the JVM args.
        
        System.out.println("Zone System:"+ System.getProperty("user.timezone")+", TimeZone.getDefault().toZoneId(): "+ZoneId.systemDefault());
        
        String formatStr = "yyyy-MM-dd'T'HH:mm:ss";
        for (String strDate : datesArr) {
            
            DateFormat dateFormat = new SimpleDateFormat(formatStr);
            dateFormat.setTimeZone(TimeZone.getTimeZone("UTC")); // Provided date as CET, Converting corresponding Default UTC date
            Date dates = dateFormat.parse(strDate);
            
            String utcDate = dateFormat.format(dates);
            
            /*SimpleTimeZone[id=Europe/Berlin,dstSavings=3600000,useDaylight=true]
                    1603591200000 : UTC TimeStamp: 2020-10-25 02:00:00.0
                    1603587600    : CET
                          3600    : dstSavings*/
            Calendar cal = new GregorianCalendar();
            cal.setTimeZone(TimeZone.getTimeZone("UTC"));
            cal.setTimeInMillis(dates.getTime());
            
            
            DateFormat dateFormat22 = new SimpleDateFormat(formatStr);
            dateFormat22.setTimeZone(TimeZone.getTimeZone("CET")); // UTC, -01:00, GMT, GMT-7, IST-"Asia/Kolkata"
            Date dates22 = dateFormat22.parse(utcDate);
            
            DateFormat dateFormat2 = new SimpleDateFormat(formatStr, Locale.ENGLISH);
            dateFormat2.setTimeZone(TimeZone.getTimeZone("CET"));
            Date dates2 = dateFormat2.parse(strDate);
            
            displayKeyVal("Privided", strDate);
            displayKeyVal("convertUTCtoCET", convertUTCtoCET(strDate) );
            
            displayKeyVal("Locale.UTC", dates+"");
            displayKeyVal("UTC Str", utcDate);
            displayKeyVal("GMT String", dates.toGMTString());
            displayKeyVal("Locale.CET", dates22+"");
            displayKeyVal("Locale.ENGLISH", dates2+"");
            
            displayKeyVal("UTC Time Default(CET)", new java.sql.Timestamp(dates.getTime())+"");
            displayKeyVal("UTC - CET Time", new java.sql.Timestamp(dates22.getTime())+"");
            displayKeyVal("ENGLISH Time", new java.sql.Timestamp(dates2.getTime())+"");
            
            displayKeyVal("GregorianCalendar", cal.getTime()+"");
            displayKeyVal("TimeStamp GC", new java.sql.Timestamp(cal.getTimeInMillis())+"");
            
            displayKeyVal("TimeStamp Local", (getTimestamp(strDate)).toString());
            System.out.println("======================");
        }
        
        /*String startDate = "2020-03-29T01:00:00";
        String endDate = "2020-03-29T02:00:00";
        compareTwoTimeStamps(startDate, endDate);
        
        String startDate2 = "2020-03-29T03:00:00";
        String endDate2 = "2020-03-29T04:00:00";
        compareTwoTimeStamps(startDate2, endDate2);
        
        System.out.println("------------------------------");
        try {
            lenientStrictParseDate(dateStr, "UTC");
        } catch (ParseException e) {
            e.printStackTrace();
        }
        
        try {
            parseDate(dateStr);
        } catch (ParseException e) {
            e.printStackTrace();
        }
        
        Instant instant = Instant.parse( dateStr + "Z"); // `Instant` is always in UTC.
        java.util.Date date = java.util.Date.from( instant );
        System.out.println("UTC : "+date);
        
        String prop = System.getProperty("java.time.zone.DefaultZoneRulesProvider");
        System.out.println("DefaultZoneRulesProvider : "+ prop);
        
        displayZonedDate(dateStr, "UTC"); // UTC, GMT, UT
        displayZonedDate(dateStr, "CET");
        //displayZonedDate(dateStr, "CETDST");
        
        // displayZoneRules(); */
    }
    
    public static Timestamp getTimestamp(String strDate) {
        DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd'T'HH:mm:ss");
        LocalDateTime dateTime = LocalDateTime.parse(strDate, formatter);
                                //LocalDateTime.of(10000, Month.DECEMBER, 30, 12, 10, 05);
        
        Timestamp valueOf = java.sql.Timestamp.valueOf(dateTime);
        System.out.format("TimeStamp Local : [%-30s]\n", valueOf.toString());
        
        java.sql.Date sqlDate = new java.sql.Date( valueOf.getTime() );
        System.out.format("java.sql.Date : [%-30s]\n", sqlDate);
        
        return valueOf;
        
        /*LocalDateTime with = dateTime.with(TemporalAdjusters.lastInMonth(DayOfWeek.SUNDAY));
        int dayOfMonth = with.getDayOfMonth();
        System.out.println("Last Sunday of Month: "+dayOfMonth);*/
    }
    public static void displayKeyVal(String key, String val) {
        System.out.format("[%-25s] : [%-35s]\n", key, val);
    }
    
    public static long compareTwoTimeStamps(String startDate, String endDate) throws ParseException {
        Date start = parseDate(startDate);
        Date end = parseDate(endDate);
        
        java.sql.Timestamp startTime = getTimeStamp(start);
        java.sql.Timestamp endTime = getTimeStamp(end);
        
        long startTimeMilli = startTime.getTime();
        long endTimeMilli = endTime.getTime();
        long diff = endTimeMilli - startTimeMilli;
        
        //long diffSeconds = diff / 1000;
        //long diffMinutes = diff / (60 * 1000);
        long diffHours = diff / (60 * 60 * 1000);
        //long diffDays = diff / (24 * 60 * 60 * 1000);
        
        System.out.format("Start:%s, End:%s\n", start, end);
        System.out.println("Difference in Hours : "+ diffHours);
        if (diffHours >= 1) {
            Timestamp oneHourAgo = new Timestamp(endTime.getTime() - (60 * 60 * 1000));
            System.out.println("--- Before: "+ endTime.toLocaleString());
            System.out.println("--- After: "+ oneHourAgo.toLocaleString());
        }
        return diffHours;
    }
    
    
    public static void displayZonedDate(String dateStr, String zone) {
        Instant instant = Instant.parse( dateStr + "Z");
        ZoneId zoneId = ZoneId.of( zone ); // Define a time zone rather than rely implicitly on JVM’s current default time zone.
        ZonedDateTime zdt = ZonedDateTime.ofInstant( instant , zoneId );  // Assign a time zone adjustment from UTC.
        java.util.Date dateZoned = java.util.Date.from( zdt.toInstant() );
        System.out.println(zone+" : "+dateZoned);
    }
    
    public static Timestamp getTimeStamp(Date date) { //Date dbResponseTime = new java.util.Date();
        Timestamp timestamp = new java.sql.Timestamp(date.getTime());
        System.out.println("Timestamp :"+ timestamp);
        return timestamp;
    }
    
    private static String convertUTCtoCET(String UTCDate) throws ParseException {
        SimpleDateFormat formatter = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss");
        Date date = formatter.parse(UTCDate);
        
        SimpleDateFormat FORMATTER = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        String dateValue = FORMATTER.format(date);
        
        SimpleDateFormat DF = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        DF.setLenient(true);
        DF.setTimeZone(TimeZone.getTimeZone("UTC"));
        SimpleDateFormat DF1 = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
        Date startDate = DF.parse(dateValue);
        
        DF1.setTimeZone(TimeZone.getTimeZone("CET"));
        String CETDateFormat = DF1.format(startDate);
        return CETDateFormat;
    }
    
    
    static String formatStr = "yyyy-MM-dd'T'HH:mm:ss";
    public static Date lenientStrictParseDate(String aSource, String zone) throws ParseException {
        DateFormat dateFormat = new SimpleDateFormat(formatStr);
        dateFormat.setTimeZone(TimeZone.getTimeZone(zone));
        //dateFormat.setLenient(false);
        Date date = dateFormat.parse(aSource);
        System.out.format(zone + " Old [%s]= New Date [%s]\n", aSource, date.toGMTString());
        
        getTimeStamp(date);
        
        return date;
    }
    public static Date parseDate(String aSource) throws ParseException {
        DateFormat dateFormat = new SimpleDateFormat(formatStr, Locale.ENGLISH);
        Date date = dateFormat.parse(aSource);
        System.out.format("attribute [%s]= Date [%s]\n", aSource, date.toString());
        
        getTimeStamp(date);
        
        return date;
    }
    
    public static void displayZoneRules() {
        ZoneId zone = ZoneId.of("CET");
        System.out.println(zone);
        System.out.println(zone.getRules());
        for (ZoneOffsetTransition trans : zone.getRules().getTransitions()) {
          System.out.println(trans);
        }
        for (ZoneOffsetTransitionRule rule : zone.getRules().getTransitionRules()) {
          System.out.println(rule);
        }
    }
}