Develop Reference

Changing date formats

I have the following code:
// OLD DATE
String date = "Mon, 06/07";
DateFormat df = new SimpleDateFormat("MM/dd");
String strDate = date.substring(date.length() - 5);
Date dateOld;
try {
dateOld = df.parse(strDate);
} catch (Exception e) {
e.printStackTrace();
}
String dateStr = df.format(dateOld);
MonthDay monthDay = MonthDay.parse(dateStr, DateTimeFormatter.ofPattern("MM/dd"));
ZonedDateTime dateNew = ZonedDateTime.now().with(monthDay);
// NEW DATE
System.out.println(dateNew.format(DateTimeFormatter.ofPattern("uuuu-MM-dd'T00:00:00Z'")));
Basically what I am trying to do is change Mon, 06/07 format to this format 2021-06-07T00:00:00Z.
What I have works, but it is really terrible. What would be a better way of doing it?

This is a little tricky as you need to make some assumptions
The year, as it's not specified in the original format
TimeZone, as it's not specified at all (the final output seems to point to UTC)
The first thing your need to do, is parse the String input into a LocalDate (you could just go straight to ZonedDate, but this is where I started)
String date = "Mon, 06/07";
DateTimeFormatter parseFormatter = new DateTimeFormatterBuilder()
.appendPattern("E, M/d")
.parseDefaulting(ChronoField.YEAR, 2021)
.toFormatter(Locale.US);
LocalDate ld = LocalDate.parse(date, parseFormatter);
Then you need to convert that to LocalDateTime
LocalDateTime ldt = ld.atStartOfDay();
And then, to a ZonedDateTime. Here' I've assumed UTC
//ZoneId zoneId = ZoneId.systemDefault();
//ZonedDateTime zdt = ldt.atZone(zoneId);
OffsetDateTime zdt = ldt.atOffset(ZoneOffset.UTC);
And finally, format the result to your desired format
String formatted = zdt.format(DateTimeFormatter.ISO_OFFSET_DATE_TIME);
System.out.println(formatted);
Which, for me, prints...
2021-06-07T00:00:00Z
A lot of time and effort has gone into the new Date/Time APIs and you should make the time to try and learn them as best you can (I'm pretty rusty, but with a little tinkering, got to a result)
Maybe start with Date/Time trails

A solution use Calendar, Date and SimpleDateFormat
SimpleDateFormat sdf = new SimpleDateFormat("EEE, MM/dd", Locale.getDefault());
try {
Date oldDate = sdf.parse("Mon, 06/07");
Calendar calendar = Calendar.getInstance();
int savedYear = calendar.get(Calendar.YEAR);
if (oldDate != null) {
calendar.setTime(oldDate);
calendar.set(Calendar.YEAR, savedYear);
sdf.applyPattern("yyyy-MM-dd'T00:00:00Z'");
System.out.println(sdf.format(calendar.getTime()));
}
} catch (ParseException e) {
e.printStackTrace();
}

Java Date Parsing from string

I'm trying to parse a String into Data, I create the DataParser, in according to date format, the code I wrote is this:
String date_s = "04-May-2017 17:28:27";
DateFormat formatter = new SimpleDateFormat("dd-MMM-yyyy HH:mm:ss");
Date date;
try {
date = formatter.parse(date_s);
System.out.println(date);
} catch (ParseException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
When I execute this, I got always an exception
java.text.ParseException: Unparseable date: "04-May-2017 17:28:27"
I don't understand why the data is not parsed, someone can help me?

This thread of answers would not be complete without the modern solution. These days you should no longer use Date and SimpleDateFormat, but switch over to the newer date and time classes:
DateTimeFormatter formatter
= DateTimeFormatter.ofPattern("dd-MMM-yyyy HH:mm:ss", Locale.ENGLISH);
LocalDateTime dateTime;
try {
dateTime = LocalDateTime.parse(date_s, formatter);
System.out.println(dateTime);
} catch (DateTimeParseException e) {
System.err.println(e);
}
This prints
2017-05-04T17:28:27
(LocalDateTime.toString() returns ISO 8601 format) If leaving out Locale.ENGLISH, on my computer I get
Text '04-May-2017 17:28:27' could not be parsed at index 3
Index 3 is where it say May, so the message is somewhat helpful.
LocalDateTime and DateTimeFormatter were introduced in Java 8, but have also been backported to Java 6 and 7.

the string you want to parse is local dependent (the word May is English), so the jvm is not able to infer that may is the month of may in English
define the formatter using the constructor qith the locale.
DateFormat formatter = new SimpleDateFormat("dd-MMM-yyyy HH:mm:ss",Locale.ENGLISH);

You need another constructor with a Locale that supports MMM (May)
DateFormat formatter = new SimpleDateFormat("dd-MMM-yyyy HH:mm:ss",Locale.US)
or using standard format dd-MM-yyyy with month digits.
(Sorry, in the meantime the answer was already posted)

SimpleDateFormat.parse() - generates wrong date for different date-formats

Below is my code to parse the date using SimpleDateFormat with pattern:
String pattern = "yyyy-MM-dd";
SimpleDateFormat format = new SimpleDateFormat(pattern);
try {
Date date = format.parse("05-21-2030");
System.out.println(date);
} catch (ParseException e) {
e.printStackTrace();
}
You can see the date which I passed to parse is different from date format which is specified in SimpleDateFormat. In this case I was expecting kind of excpetion as format is different but it parsed successfully with some different date values. I got the output - Tue Mar 22 00:00:00 IST 12
When I pass the same format like 2030-05-21 it works fine.
Can you guys please let me know how can I prevent such things in my code?

Basically you want SimpleDateFormat to be strict, so set lenient to false.
SimpleDateFormat format = new SimpleDateFormat(pattern);
format.setLenient(false);

If you can afford using Java 8 time API, its formatter works as expected:
DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd");
try {
LocalDate date = LocalDate.parse("2030-05-21", formatter);
System.out.println(date);
LocalDate date2 = LocalDate.parse("05-21-2030", formatter);
System.out.println(date2);
} catch (DateTimeParseException e) {
e.printStackTrace();
}
Output:
2030-05-21
java.time.format.DateTimeParseException: Text '05-21-2030' could not be parsed at index 0
at java.time.format.DateTimeFormatter.parseResolved0(DateTimeFormatter.java:1947)
at java.time.format.DateTimeFormatter.parse(DateTimeFormatter.java:1849)
at java.time.LocalDate.parse(LocalDate.java:400)
at java8.Snippet.main(Snippet.java:25)

Parsing a date in Korean

I am trying to parse a date in Korean Date format using SimpleDateFormat which works. However i would like to remove any dependency on adding up Korean Characters in Pattern for Year(년), Month(월) and Day(일) and so on.
String dateinKorean = "2013년 9월 26일 (목)";
SimpleDateFormat sdf = new SimpleDateFormat("yyyy'년' M'월' d'일' '('EE')'", Locale.KOREA);
try {
Date dt = sdf.parse(dateinKorean);
System.out.println(dt.toGMTString());
} catch (ParseException e) {
e.printStackTrace();
}
I am trying to use DateFormatSymbols Class to Parse the date using the Locale, however the problem is I am not able to parse the complete date, I can parse an individual Month(MM), Year(yyyy) or Day(dd) without any issues.
DateFormatSymbols df = DateFormatSymbols.getInstance(Locale.KOREAN);
SimpleDateFormat sdf = new SimpleDateFormat("yyyy MM dd", df);
try {
// 2013년 9월 26일
Date dt = sdf.parse("2013년 9월 26일");
} catch (ParseException e) {
}
Can anyone please help me identify if there is any other way to parse the dates other than shown above ?

If the date format includes "fixed" characters they are used as-is, so if you include words like "year" or "month" in the format they cannot be ignored.
You can hack your way around this by removing all non-digits from the string before parsing, for example:
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd", df);
Date dt = sdf.parse("2013년 9월 26일".replaceAll("\\D+", "-"));
A better solution could be internationalizing the date format so that each language you support could have a format of its own.

Calendar date to yyyy-MM-dd format in java

How to convert calendar date to yyyy-MM-dd format.
Calendar cal = Calendar.getInstance();
cal.add(Calendar.DATE, 1);
Date date = cal.getTime();
SimpleDateFormat format1 = new SimpleDateFormat("yyyy-MM-dd");
String date1 = format1.format(date);
Date inActiveDate = null;
try {
inActiveDate = format1.parse(date1);
} catch (ParseException e1) {
// TODO Auto-generated catch block
e1.printStackTrace();
}
This will produce inActiveDate = Wed Sep 26 00:00:00 IST 2012. But what I need is 2012-09-26. My purpose is to compare this date with another date in my database using Hibernate criteria. So I need the date object in yyyy-MM-dd format.

A Java Date is a container for the number of milliseconds since January 1, 1970, 00:00:00 GMT.
When you use something like System.out.println(date), Java uses Date.toString() to print the contents.
The only way to change it is to override Date and provide your own implementation of Date.toString(). Now before you fire up your IDE and try this, I wouldn't; it will only complicate matters. You are better off formatting the date to the format you want to use (or display).
Java 8+
LocalDateTime ldt = LocalDateTime.now().plusDays(1);
DateTimeFormatter formmat1 = DateTimeFormatter.ofPattern("yyyy-MM-dd", Locale.ENGLISH);
System.out.println(ldt);
// Output "2018-05-12T17:21:53.658"
String formatter = formmat1.format(ldt);
System.out.println(formatter);
// 2018-05-12
Prior to Java 8
You should be making use of the ThreeTen Backport
The following is maintained for historical purposes (as the original answer)
What you can do, is format the date.
Calendar cal = Calendar.getInstance();
cal.add(Calendar.DATE, 1);
SimpleDateFormat format1 = new SimpleDateFormat("yyyy-MM-dd");
System.out.println(cal.getTime());
// Output "Wed Sep 26 14:23:28 EST 2012"
String formatted = format1.format(cal.getTime());
System.out.println(formatted);
// Output "2012-09-26"
System.out.println(format1.parse(formatted));
// Output "Wed Sep 26 00:00:00 EST 2012"
These are actually the same date, represented differently.

Your code is wrong. No point of parsing date and keep that as Date object.
You can format the calender date object when you want to display and keep that as a string.
Calendar cal = Calendar.getInstance();
cal.add(Calendar.DATE, 1);
Date date = cal.getTime();
SimpleDateFormat format1 = new SimpleDateFormat("yyyy-MM-dd");
String inActiveDate = null;
try {
inActiveDate = format1.format(date);
System.out.println(inActiveDate );
} catch (ParseException e1) {
// TODO Auto-generated catch block
e1.printStackTrace();
}

java.time
The answer by MadProgrammer is correct, especially the tip about Joda-Time. The successor to Joda-Time is now built into Java 8 as the new java.time package. Here's example code in Java 8.
When working with date-time (as opposed to local date), the time zone in critical. The day-of-month depends on the time zone. For example, the India time zone is +05:30 (five and a half hours ahead of UTC), while France is only one hour ahead. So a moment in a new day in India has one date while the same moment in France has “yesterday’s” date. Creating string output lacking any time zone or offset information is creating ambiguity. You asked for YYYY-MM-DD output so I provided, but I don't recommend it. Instead of ISO_LOCAL_DATE I would have used ISO_DATE to get this output: 2014-02-25+05:30
ZoneId zoneId = ZoneId.of( "Asia/Kolkata" );
ZonedDateTime zonedDateTime = ZonedDateTime.now( zoneId );
DateTimeFormatter formatterOutput = DateTimeFormatter.ISO_LOCAL_DATE; // Caution: The "LOCAL" part means we are losing time zone information, creating ambiguity.
String output = formatterOutput.format( zonedDateTime );
Dump to console…
System.out.println( "zonedDateTime: " + zonedDateTime );
System.out.println( "output: " + output );
When run…
zonedDateTime: 2014-02-25T14:22:20.919+05:30[Asia/Kolkata]
output: 2014-02-25
Joda-Time
Similar code using the Joda-Time library, the precursor to java.time.
DateTimeZone zone = new DateTimeZone( "Asia/Kolkata" );
DateTime dateTime = DateTime.now( zone );
DateTimeFormatter formatter = ISODateTimeFormat.date();
String output = formatter.print( dateTime );
ISO 8601
By the way, that format of your input string is a standard format, one of several handy date-time string formats defined by ISO 8601.
Both Joda-Time and java.time use ISO 8601 formats by default when parsing and generating string representations of various date-time values.

java.util.Date object can't represent date in custom format instead you've to use SimpleDateFormat.format method that returns string.
String myString=format1.format(date);

public static void main(String[] args) {
Calendar cal = Calendar.getInstance();
cal.set(year, month, date);
SimpleDateFormat format1 = new SimpleDateFormat("yyyy MM dd");
String formatted = format1.format(cal.getTime());
System.out.println(formatted);
}

In order to parse a java.util.Date object you have to convert it to String first using your own format.
inActiveDate = format1.parse( format1.format(date) );
But I believe you are being redundant here.

Calendar cal = Calendar.getInstance();
cal.add(Calendar.DATE, 7);
Date date = c.getTime();
SimpleDateFormat ft = new SimpleDateFormat("MM-dd-YYYY");
JOptionPane.showMessageDialog(null, ft.format(date));
This will display your date + 7 days in month, day and year format in a JOption window pane.

public static String ThisWeekStartDate(WebDriver driver) {
Calendar c = Calendar.getInstance();
//ensure the method works within current month
c.set(Calendar.DAY_OF_WEEK, Calendar.SUNDAY);
System.out.println("Before Start Date " + c.getTime());
Date date = c.getTime();
SimpleDateFormat dfDate = new SimpleDateFormat("dd MMM yyyy hh.mm a");
String CurrentDate = dfDate.format(date);
System.out.println("Start Date " + CurrentDate);
return CurrentDate;
}
public static String ThisWeekEndDate(WebDriver driver) {
Calendar c = Calendar.getInstance();
//ensure the method works within current month
c.set(Calendar.DAY_OF_WEEK, Calendar.SATURDAY);
System.out.println("Before End Date " + c.getTime());
Date date = c.getTime();
SimpleDateFormat dfDate = new SimpleDateFormat("dd MMM yyyy hh.mm a");
String CurrentDate = dfDate.format(date);
System.out.println("End Date " + CurrentDate);
return CurrentDate;
}

I found this code where date is compared in a format to compare with date field in database...may be this might be helpful to you...
When you convert the string to date using simpledateformat, it is hard to compare with the Date field in mysql databases.
So convert the java string date in the format using select STR_to_DATE('yourdate','%m/%d/%Y') --> in this format, then you will get the exact date format of mysql date field.
http://javainfinite.com/java/java-convert-string-to-date-and-compare/

My answer is for kotlin language.
You can use SimpleDateFormat to achieve the result:
val date = Date(timeInSec)
val formattedDate = SimpleDateFormat("yyyy-MM-dd", Locale("IN")).format(date)
for details click here.
OR
Use Calendar to do it for you:
val dateObject = Date(timeInMillis)
val calendarInstance = Calendar.getInstance()
calendarInstance.time = dateObject
val date = "${calendarInstance.get(Calendar.YEAR)}-${calendarInstance.get(Calendar.MONTH)}-${calendarInstance.get(Calendar.DATE)}"
For more details check this answer.

I don't know about y'all, but I always want this stuff as a one-liner. The other answers are fine and dandy and work great, but here is it condensed to a single line. Now you can hold less lines of code in your mind :-).
Here is the one Liner:
String currentDate = new SimpleDateFormat("yyyy-MM-dd").format(new Date());