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Change date from MM/dd to yyyy-MM-dd'T'HH:mm:ss.SSZ in Java

I have a String that formatted MM/dd. I would like to convert it to a Date in format yyyy-MM-dd'T'HH:mm:ss.SSZ.
DateFormat df = new SimpleDateFormat("MM/dd");
String strDate = "06/05";
Date date = new Date();
date = df.parse(strDate);
This makes it a Date, but in the original format.
System.out.println(new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSZ").format(date));
This returns the correct month and day, but nopthing else.
1970-06-05T00:00:00.00-0400
Any idea how I can make it return
CURRENT_YEAR-06-05TCURRENT_TIME

In the question, the date format pattern indicates a desire for 2-digit fractional seconds. SimpleDateFormat cannot do that.
The newer Java 8 Time API can, and you should be using that anyway.
If you're running on Java 6 or 7, get the ThreeTen-Backport library.
To parse a MM/dd formatted string and get a full timestamp with current year and time-of-day, in the default time zone, use the following code:
String strDate = "06/05";
MonthDay monthDay = MonthDay.parse(strDate, DateTimeFormatter.ofPattern("MM/dd"));
ZonedDateTime date = ZonedDateTime.now().with(monthDay);
System.out.println(date.format(DateTimeFormatter.ofPattern("uuuu-MM-dd'T'HH:mm:ss.SSZ")));
Sample Output
2020-06-05T14:52:48.45-0400

I recommend to make use of java.time package. There you go:
var ds = "01/12";
var df = java.time.format.DateTimeFormatter.ofPattern("MM/dd");
var dt = java.time.MonthDay.from(df.parse(ds)).adjustInto(java.time.LocalDateTime.now());
Then you can convert dt to java.util.Date or whatever you like. Or simply use one of java.time formaters to get the desired output.

You are creating a date with only month and day
If you want to use the current year and time, you can create a calendar object and edit the month and day

For something this simple I suggest a different approach, get current time then set month and day from the original string THEN format.
String str = "08/09";
String[] split = str.split("/");
Calendar calendar = Calendar.getInstance();
calendar.set(Calendar.MONTH, Integer.parseInt(split[0]));
calendar.set(Calendar.DAY_OF_MONTH, Integer.parseInt(split[1]));
System.out.println(new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss.SSS").format(calendar.getTime()));

String strDate = "06-05";
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-"+strDate+"'T'HH:mm:ss.SSZ");
System.out.println(sdf.format(new Date()));
the output:
2020-06-05T23:00:45.306+0400

Date selector based on months in Java

I have code in Java for Selenium Webdriver TestNG. Is for comparing if the date on the website is same as today date.
Problem is that date on webpage dateOnWebpage for 11th April 2018 is in format
Today 4/11/2018
So I made selecter to compare date formats if months < 10 if(javaDateSelector < 10) than date to compare is in format M/dd/yyyy else is in format MM/dd/yyyy.
Is there better way to code it than I made it? Because I needed to parse date to string and than to int to compare it and code is quite long.
#Test(priority=3)
public void test3DateCheck() throws Exception
{
String dateOnWebpage = driver.findElement(By.xpath("//div[#id='homeCalendarSection']/div/div[2]/table/tbody/tr/td/div/ul/li")).getText();
System.out.println("Today Date on webpage is : " + dateOnWebpage);
//DateFormat dateFormat1 = new SimpleDateFormat("M/dd/yyyy");
DateFormat dateFormat1 = new SimpleDateFormat("MM");
Date date = new Date();
String javaDate1 = dateFormat1.format(date);
int javaDateSelector = Integer.parseInt(javaDate1);
if(javaDateSelector < 10)
{
DateFormat dateFormat2 = new SimpleDateFormat("M/dd/yyyy");
String javaDate2 = dateFormat2.format(date);
System.out.println("Today Date from Java is : " + javaDate2);
Assert.assertEquals(dateOnWebpage, "Today " + javaDate2);
}
else
{
DateFormat dateFormat3 = new SimpleDateFormat("MM/dd/yyyy");
String javaDate3 = dateFormat3.format(date);
System.out.println("Today Date from Java is : " + javaDate3);
Assert.assertEquals(dateOnWebpage, "Today " + javaDate3);
}
}

If you don’t care whether the date on the web page is written with leading zero for month and day of month or not and just want to test whether the date is correct:
DateTimeFormatter dateFormatter = DateTimeFormatter.ofPattern("'Today' M/d/uuuu");
LocalDate today = LocalDate.now(ZoneId.of("Pacific/Norfolk"));
System.out.println("Today is " + today);
LocalDate observedDate = LocalDate.parse(dateOnWebpage, dateFormatter);
Assert.assertEquals(today, observedDate);
Rather than testing the string I am parsing the date and testing it. Even though the pattern has one M and one d in it, parsing two-digit months and two-digits day of month poses no problem.
If on the other hand you also want to test that the date on the web page is written without any leading zeroes, it’s best to test the string, like you already did:
DateTimeFormatter dateFormatter = DateTimeFormatter.ofPattern("M/d/uuuu");
LocalDate today = LocalDate.now(ZoneId.of("Pacific/Norfolk"));
String todayString = today.format(dateFormatter);
System.out.println("Today is " + todayString);
Assert.assertEquals("Today " + todayString, dateOnWebpage);
Again, even though the pattern has one M and one d in it, two digits will be printed if the month or the day of month is greater than 9. What else could the format method do? If you require two-digit day of month always, put dd in the format pattern string.
In both snippets please fill in your desired time zone where I put Pacific/Norfolk since it is never the same date everywhere on the globe.
I am using and recommending java.time, the modern Java date and time API. DateFormat and SimpleDateFormat are not only long outdated, they are also notoriously troublesome. Date is just as outdated. I would avoid those classes completely. The modern API is generally so much nicer to work with.
Link: Oracle tutorial: Date Time explaining how to use java.time.

Subtract char of digits like normal integers

I have two strings which can be seen as time stamps:
String min="2017-04-15 13:27:31";
String max="2017-04-15 13:40:01";
Assume we want to find out the time passed from first time stamp to the second one. If there was only the time and no date included, I could get it using my following code:
String[] partsMin=min.split(":");
String[] partMax=max.split(":");
int diffZero=Integer.parseInt(partMax[0])-Integer.parseInt(partsMin[0]);
int diffOne=Integer.parseInt(partMax[1])-Integer.parseInt(partsMin[1]);
int diffOTwo=Integer.parseInt(partMax[2])-Integer.parseInt(partsMin[2]);
diffInSec=diffZero*3600+diffOne*60+diffOTwo;
So here is the question. How to get the job done while there is a date within the time stamp?

I would construct LocalDateTime instances from it.
Then i would get the milliseconds from it and substract startTime from EndTime.
What is remaining are the milliseconds passed between the two. A DateTimeFormatter is helpful as well for this purpose.
String strMin = "2017-04-15 13:27:31";
DateTimeFormatter formatterTime = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
LocalDateTime dateTimeMin = LocalDateTime.parse(strMin, formatter);
String strMax = "2017-04-15 13:40:01";
LocalDateTime dateTimeMax = LocalDateTime.parse(strMax, formatter);
long minutes = ChronoUnit.MINUTES.between(dateMin, dateMaxto);
long hours = ChronoUnit.HOURS.between(dateMin, dateMax);
If you want to get the milliseconds:
long millisPassed = dateMax.toEpochMilli() - dateMax.toEpochMilli();

Use the java date time libraries (even the old Date class would be fine for this) to parse the string into a proper object.
Depending on the date time library you chose you can then look at the difference between them. The simplest would be something like:
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss");
Date date1 = sdf.parse(str1);
Date date2 = sdf.parse(str2);
long differenceInSeconds = (date2.getTime()-date1.getTime())/1000;
The new Java 8 time classes would also allow you to do this and would be better to learn going forwards. I can't remember the syntax for that off the top of my head though.

Did you try with replace all the other part of your String like this :
String[] partsMin = min.replaceAll("\\d+-\\d+-\\d+", "").trim().split(":");
String[] partMax = max.replaceAll("\\d+-\\d+-\\d+", "").trim().split(":");

Doing this in your code:
int diffZero=Integer.parseInt(partMax[0])
is the same as doing:
int diffZero=Integer.parseInt("2017-04-15")
that is generating an Exception(NumberFormatException)
you should better try to PARSE those strings min and max into a date
Edit:
you can inspect your code/ variables: and see that splitting to ":" is not giving you back the correct array since the element at index 0 is holding more information than you need...
but as I said before, you are going on the wrong path, dont re invent the wheel and look how practical will get using the APIs that java has for us:
String min = "2017-04-15 13:27:31";
String max = "2017-04-15 13:40:01";
DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");
LocalDateTime dateTimeMin = LocalDateTime.parse(min, formatter);
LocalDateTime dateTimeMax = LocalDateTime.parse(max, formatter);
long days = ChronoUnit.DAYS.between(dateTimeMin, dateTimeMax);
long minutes = ChronoUnit.MINUTES.between(dateTimeMin, dateTimeMax);
System.out.println(days);
System.out.println(minutes);

use SimpleDateFormat to parse the date string, and do operation on Date result, you will get right value. This works well for date between '2017-02-28 23:59:59' and '2017-03-01 00:00:01'
SimpleDateFormat format = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
Date date1 = format.parse("2017-02-28 23:59:59");
Date date2 = format.parse("2017-03-01 00:00:01");
long time1 = date1.getTime();
long time2 = date2.getTime();
long diff = time2 - time2; // should be 2000

Difference between two dateFields in Vaadin

I'm having some problem with calculate days between two dateFields. I already tried
`
DateField dateStart = new DateField();
DateField dateEnd = new DateField();
DateTime start = new DateTime(dateStart);
DateTime end = new DateTime(dateEnd);
int days = Days.daysBetween(new DateTime(start), new DateTime(end)).getDays();
`
Here is the error that I get after run this code
java.lang.IllegalArgumentException: No instant converter found for type: com.vaadin.ui.DateField
I also already tried using ChronoUnit
LocalDate startDate = LocalDate.now().minusDays(1);
LocalDate endDate = LocalDate.now();
long days = Period.between(startDate, endDate).getDays();
assertEquals(1, days);
long days2 = ChronoUnit.DAYS.between(startDate, endDate);
assertEquals(1, days2);
and also JudoTime
DateTime startDate = new DateTime().minusDays(1);
DateTime endDate = new DateTime();
Days d = Days.daysBetween(startDate, endDate);
int days = d.getDays();
Both code I get NullPointerException error. Is there any way that I can get number of days.

The other answers show how to calculate the difference between two dates in days using Java 8. But your actual problem is how to get the dates from Vaadin DateField (as p.streef and Ramachandran G A pointed out in the comments). Use dateStart.getValue() for that which will return a java.util.Date and can be passed to new DateTime().

This is simplest way to find the intermediate date values using java 8.
LocalDate startDate = LocalDate.now().minusDays(1);
LocalDate endDate = LocalDate.now();
long days = Period.between(startDate, endDate).getDays();
System.out.println(days);

The following snippet works. One of the reason for no instant converter error that you see is what time of the day do you want the day to be at. I mentioned start of day here. Interesting read here (How to convert Joda Localdate to Joda DateTime?) .
LocalDateTime startDateTime = LocalDate.now().atStartOfDay().minusDays(1);
LocalDateTime endDateTime = LocalDate.now().atStartOfDay();
long days = Period.between(startDateTime.toLocalDate(), endDateTime.toLocalDate()).getDays();
System.out.println("The difference in dates is " + days);
At the output : The difference in dates is 1

Adding Years to a random date from Date class

Let's say I have this:
PrintStream out = System.out;
Scanner in = new Scanner(System.in);
out.print("Enter a number ... ");
int n = in.nextInt();
I have a random date, for example, 05/06/2015 (it is not a fixed date, it is random every time). If I want to take the 'year' of the this date, and add whatever 'n' is to this year, how do i do that?
None of the methods in the Date Class are 'int'.
And to add years from an int, 'years' has to be an int as well.

You need to convert the Date to a Calendar.
Calendar c = Calendar.getInstance();
c.setTime(randomDate);
c.add(Calendar.YEAR, n);
newDate = c.getTime();
You can manipulate the Year (or other fields) as a Calendar, then convert it back to a Date.

This question has long deserved a modern answer. And even more so after Add 10 years to current date in Java 8 has been deemed a duplicate of this question.
The other answers were fine answers in 2012. The years have moved on, today I believe that no one should use the now outdated classes Calendar and Date, not to mention SimpleDateFormat. The modern Java date and time API is so much nicer to work with.
Using the example from that duplicate question, first we need
private static final DateTimeFormatter formatter
= DateTimeFormatter.ofPattern("uuuu-MM-dd HH:mm:ss");
With this we can do:
String currentDateString = "2017-09-12 00:00:00";
LocalDateTime dateTime = LocalDateTime.parse(currentDateString, formatter);
dateTime = dateTime.plusYears(10);
String tenYearsAfterString = dateTime.format(formatter);
System.out.println(tenYearsAfterString);
This prints:
2027-09-12 00:00:00
If you don’t need the time of day, I recommend the LocalDate class instead of LocalDateTime since it is exactly a date without time of day.
LocalDate date = dateTime.toLocalDate();
date = date.plusYears(10);
The result is a date of 2027-09-12.
Question: where can I learn to use the modern API?
You may start with the Oracle tutorial. There’s much more material on the net, go search.

Another package for doing this exists in org.apache.commons.lang3.time, DateUtils.
Date date = new Date();
date = DateUtils.addYears(date, int quantity = 1);

The Date class will not help you, but the Calendar class can:
Calendar cal = Calendar.getInstance();
Date f;
...
cal.setTime(f);
cal.add(Calendar.YEAR, n); // Where n is int
f = cal.getTime();
Notice that you still have to assign a value to the f variable. I frequently use SimpleDateFormat to convert strings to dates.
Hope this helps you.

Try java.util.Calendar type.
Calendar cal=Calendar.getInstance();
cal.setTime(yourDate.getTime());
cal.set(Calendar.YEAR,n);

This will add 3 years to the current date and print the year.
System.out.println(LocalDate.now().plusYears(3).getYear());

If you need add one year a any date use the object Calendar.
Calendar dateMoreOneYear = Calendar.getInstance();
dateMoreOneYear.setTime(dateOriginal);
dateMoreOneYear.add(Calendar.DAY_OF_MONTH, 365);

Try like this as well for a just month and year like (June 2019)
Calendar cal = Calendar.getInstance();
cal.add(Calendar.YEAR, n); //here n is no.of year you want to increase
SimpleDateFormat format1 = new SimpleDateFormat("MMM YYYY");
System.out.println(cal.getTime());
String formatted = format1.format(cal.getTime());
System.out.println(formatted);

Try this....
String s = new SimpleDateFormat("YYYY").format(new Date(random_date_in_long)); //
int i = Integer.parseInt(s)+n;