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I need to convert new Date() to Julian date format.is there is any build in java function for this. my exact requirement is

Represents the creation date of the file in Julian date format (0YYDDD): 0 – numeric zero YY – last two digits of the year DDD – day number within the year Can be up to 7 calendar days before the date of transmission Example: 010163 = June 11, 2010

What is really looking is some thing like this 

Date date=new Date();
String JulianDtae=date.someFunction()

Any help will be appreciated

abhi
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    Look into `SimpleDateFormat` and come back should you encounter any issue with that. – dotvav Aug 19 '15 at 09:09
  • What *exactly* are you expecting for "Julian date format"? (If your variable were a `double`, that would make more sense to me...) – Jon Skeet Aug 19 '15 at 09:09
  • @Jon Skeet June 11, 2010 =010163 = this type of format – abhi Aug 19 '15 at 09:11
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    @abhi: What's the correlation between June 11th 2010 and "010163"? Is that meant to just be a number, or a formatted string? Please give more information... (A reference to the rules you're trying to implement would be useful. Please bear in mind that as I mentioned, "Julian date" and "Julian date format" have multiple meanings.) – Jon Skeet Aug 19 '15 at 09:14
  • @Jon Skeet Represents the creation date of the file in Julian date format (0YYDDD): 0 – numeric zero YY – last two digits of the year DDD – day number within the year Can be up to 7 calendar days before the date of transmission Example: 010163 = June 11, 2010 – abhi Aug 19 '15 at 09:22
  • Related question: http://stackoverflow.com/questions/1171208/what-is-the-precise-definition-of-jdes-julian-date-format. Answers include conversion code in various languages that should be trivial tor translate into Java. – Stephen C Aug 19 '15 at 09:29
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    @abhi: Right. It would have been useful to include that in the question. – Jon Skeet Aug 19 '15 at 09:34
  • Actually, June 11 is the 192nd day of 2010, not the 163rd one. – clapsus Aug 19 '15 at 13:45
  • @clapsus A typo? It’s the 162nd day of that year (in a leap year June 11 is the 163rd day, but 2010 was no leap year). – Ole V.V. Jan 20 '21 at 17:51
  • Update: The `Date` class seen in this Question is now legacy, supplanted by the *java.time* classes defined in JSR 310. For a modern solution, see the [Answer by Ole V.V.](https://stackoverflow.com/a/65814593/642706). – Basil Bourque Jan 21 '21 at 07:59

3 Answers3

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Use SimpleDateFormat.

The following code returns the Julian date string for date according to the format you gave.

String julianDateString = new SimpleDateFormat("'0'yyD").format(date);
clapsus
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java.time

I recommend that you use java.time, the modern Java date and time API, for your date work. The format you need is built in.

    LocalDate today = LocalDate.now(ZoneId.systemDefault());
    String ordinalDateString = today.format(DateTimeFormatter.ISO_ORDINAL_DATE);
    System.out.println(ordinalDateString);

Output for today January 20, 2021 in standard ISO 8601 format:

2021-020

The format you mention, 0YYDDD, is peculiar. It’s nothing I have seen before. If you’re serious about it, define a formatter that gives it:

    DateTimeFormatter peculiarDateFormatter = DateTimeFormatter.ofPattern("0uuDDD");

021020

Ordinal date, not Julian date

The day number of the year that you ask for is called the ordinal date, which is why the built-in formatter also has ordinal in its name. A Julian day is something else, the continuous count of days since January 1, 4713 BCE. The ordinal date is sometimes referred to as Julian, but there is nothing genuinely Julian about it, so to avoid confusion, prefer ordinal over Julian.

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Basil Bourque
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Ole V.V.
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0

Actually I think what you need is

String yearYy = new SimpleDateFormat("yy").format(today)
String dayD = new SimpleDateFormat("D").format(today)
String dayDDD = dayD.padLeft(3,'0')
String julianDateString = yearYy + dayDDD

This gives the proper Julian date format - there shouldn't be a leading '0', but you do need to pad the day number so that it's always 3 characters.

...I'm quite sure this could be simplified, but the important thing is that the day number should be padded.

So 20/01/21 gives 21020 (rather than 02120 when using the previous example)

ianf
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    I recommend you don’t use `SimpleDateFormat` and `Date`. Those classes are poorly designed and long outdated, the former in particular notoriously troublesome. Instead use `LocalDate` and `DateTimeFormatter`, both from [java.time, the modern Java date and time API](https://docs.oracle.com/javase/tutorial/datetime/). – Ole V.V. Jan 20 '21 at 17:22