Convert month and day number to day of year using CALENDAR

Question

I have two numbers. First would be the month number. Second would be the day number.

I also know the year.

How can I take those two number, month and day and make it into a single number DAY_OF_YEAR?

I'm using Java 1.7 and the Calendar functions.

Answer 1

java.time through ThreeTen Backport

    int year = 2019;
    int monthNumber = 9;
    int dayNumber = 28;

    LocalDate date = LocalDate.of(year, monthNumber, dayNumber);
    int dayOfYear = date.getDayOfYear();

    System.out.println("Day of year is " + dayOfYear);

Output from this snippet is:

Day of year is 271

Tested on Java 1.7.0_79 using ThreeTen Backport 1.3.6 and importing org.threeten.bp.LocalDate .

Consider avoiding the Calendar class

Four lines of the currently accepted answer creating and setting the Calendar object are substituted by just one line here. It's typical for code using Calendar to be so wordy, which we shouldn't want. Also despite the name a Calendar object is more than a calendar date, it also carries with it a time of day, a time zone and more, so I do not consider it a good fit for the job at hand. The code using Calendar will give a different result if the default locale is Thai, which will surprise most. The Calendar class is poorly designed and long outdated.

Instead I am using LocalDate of java.time, the modern Java date and time API.

Question: Can I use java.time on Java 7?

I'm using Java 1.7 …

java.time works nicely on Java 7. It just requires at least Java 6 .

• In Java 8 and later and on newer Android devices (from API level 26) the modern API comes built-in.
• In Java 6 and 7 get the ThreeTen Backport, the backport of the modern classes (ThreeTen for JSR 310; see the links at the bottom).
• On (older) Android use the Android edition of ThreeTen Backport. It's called ThreeTenABP. And make sure you import the date and time classes from org.threeten.bp with subpackages.

Links

• Oracle tutorial: Date Time explaining how to use java.time .
• Java Specification Request (JSR) 310 , where java.time was first described.
• ThreeTen Backport project , the backport of java.time to Java 6 and 7 (ThreeTen for JSR-310).
• ThreeTenABP , Android edition of ThreeTen Backport
• Question: How to use ThreeTenABP in Android Project , with a very thorough explanation.

Answer 2

Just using calendar and assuming by number of month you mean the zero-indexed one where 0 means January, here is an example for May 13th 2019:

Calendar c = Calendar.getInstance(TimeZone.getTimeZone("GMT"), Locale.ENGLISH);
c.set(Calendar.YEAR, 2019);
c.set(Calendar.MONTH, 4);
c.set(Calendar.DAY_OF_MONTH, 13);
System.out.println(c.get(Calendar.DAY_OF_YEAR));

Edit: As Ole VV's answer pointed out you get a Calendar object with the system's timezone if you call 'Calendar.getInstance()', so I changed it that way that you explicitly specify the timezone and Locale to be used. The latter is important if you eg want to get a date's week number where the rules differ in different regions of the world.

Answer 3

You can concat your parts into a String , and use a SimpleDateFormatter like:

int year = 2019;
int month = 1;
int day = 23;

String string = "" + year + "-" + month + "-" + day;
DateFormat format = new SimpleDateFormat("yyyy-MM-dd");
Date date = format.parse(string);
Calendar calendar = Calendar.getInstance();
calendar.setTime(date);
System.out.println(calendar.get(Calendar.DAY_OF_YEAR));

The example is for 2019 January 23.