Y Returns 2012 While Y Returns 2011 in Simpledateformat

Y returns 2012 while y returns 2011 in SimpleDateFormat

week year and year. From javadoc

A week year is in sync with a WEEK_OF_YEAR cycle. All weeks between
the first and last weeks (inclusive) have the same week year value.
Therefore, the first and last days of a week year may have different
calendar year values.

For example, January 1, 1998 is a Thursday. If getFirstDayOfWeek() is
MONDAY and getMinimalDaysInFirstWeek() is 4 (ISO 8601 standard
compatible setting), then week 1 of 1998 starts on December 29, 1997,
and ends on January 4, 1998. The week year is 1998 for the last three
days of calendar year 1997. If, however, getFirstDayOfWeek() is
SUNDAY, then week 1 of 1998 starts on January 4, 1998, and ends on
January 10, 1998; the first three days of 1998 then are part of week
53 of 1997 and their week year is 1997.

Facing issue in SimpleDateFormat

You need to use new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss", Locale.ENGLISH);

"YYYY" is week-based calendar year.

"yyyy" is ordinary calendar year.

You can read this if you want to know more :

https://dev.to/shane/yyyy-vs-yyyy-the-day-the-java-date-formatter-hurt-my-brain-4527

Difference between 'yy' and 'YY' in Java Time Pattern

yy is the calendar year, while YY is a week year. A week year can be different from the calendar year depending on which day the first of January falls. For example see ISO-8601 week year.

Today (30 December 2019) is a good example, the calendar year is 2019, but the week year is 2020, because this week is week 1 of 2020. So yy will result in 19, while YY results in 20.

The definition of the first week of a year from the wikipedia page:

The ISO 8601 definition for week 01 is the week with the Gregorian
year's first Thursday in it. The following definitions based on
properties of this week are mutually equivalent, since the ISO week
starts with Monday:

• It is the first week with a majority (4 or more) of its days in January.
• Its first day is the Monday nearest to 1 January.
• It has 4 January in it. Hence the earliest possible first week extends from Monday 29 December (previous Gregorian year) to Sunday 4
  January, the latest possible first week extends from Monday 4 January
  to Sunday 10 January.
• It has the year's first working day in it, if Saturdays, Sundays and 1 January are not working days.

If 1 January is on a Monday, Tuesday, Wednesday or Thursday, it is in
week 01. If 1 January is on a Friday, it is part of week 53 of the
previous year. If it is on a Saturday, it is part of the last week of
the previous year which is numbered 52 in a common year and 53 in a
leap year. If it is on a Sunday, it is part of week 52 of the previous
year.

Some locales, like for example the US, don't follow ISO-8601, because there they use Sunday as the first day of the week, but they have similar rules for week years.

SimpleDateFormat(dd-MMM-YYYY) printing year one year ahead

You're using YYYY, which is the "ISO-8601 week year". That should almost always be used in conjunction with w, "week in year". You want yyyy to show the normal calendar year.

The reason they're different is that the first ISO-8601 week of a year is the first (Monday to Sunday) week that includes at least 4 days. This means that the first week of the year is the one containing the first Thursday. As January 1st 2015 falls on a Thursday, that means that the week of 2014-12-29 to 2015-01-04 is all "week year 2015, week of year 1". (I'm surprised if you see if for December 28th...)

In other years, the first few days of the year are in week 52 or 53 of the previous year. For example, January 1st 2010 was in week 53 of week-year 2009, and January 1st 2011 was in week 52 of week-year 2010.

How does Java week year really work?

It's simple: December 27 2015 is day 1 of week 1 of week-year 2016 (and December 27 2026 is day 1 of week 1 of week-year 2027). This can be verified by adding these lines:

SimpleDateFormat odf = new SimpleDateFormat("YYYY-ww-u");
System.out.println(odf.format(d1));
System.out.println(odf.format(d2));
System.out.println(odf.format(d3));

If a SimpleDateFormat outputs a date it can use all fields: year, month, day, day of week, week of month, week in year, week-year etc.

On parsing, SimpleDateFormat expects a matching set of values: either day, month, year or day of week, week in year, week-year. Since you supplied a week-year but did not supply day of week and week in year, those to values have been assumed as 1.

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The actual values depend on your locale:

• which week of a year is week 1
• which day is the first day of the week

(see https://docs.oracle.com/javase/7/docs/api/java/util/GregorianCalendar.html#week_and_year)

On my system (using de-ch locale, with "EEE MMM dd HH:mm:ss zzz yyyy - YYYY-ww-u" as format) I get

Mo Jan 04 00:00:00 MEZ 2016 - 2016-01-1
Mo Jan 04 00:00:00 MEZ 2016 - 2016-01-1
Mo Jan 04 00:00:00 MEZ 2027 - 2027-01-1

SimpleDateFormat with Calendar giving wrong date

Replace

val myFormat = "dd/MM/YYYY HH:mm:ss"

To

val myFormat = "dd/MM/yyyy HH:mm:ss"

Java SimpleDateFormat shifts Date by one year

YYYY is the week-year, not the calendar year. You want yyyy instead. Here's Java's relevant details:

Week Of Year and Week Year

Values calculated for the WEEK_OF_YEAR field range from 1 to 53. The
first week of a calendar year is the earliest seven day period
starting on getFirstDayOfWeek() that contains at least
getMinimalDaysInFirstWeek() days from that year. It thus depends on
the values of getMinimalDaysInFirstWeek(), getFirstDayOfWeek(), and
the day of the week of January 1. Weeks between week 1 of one year and
week 1 of the following year (exclusive) are numbered sequentially
from 2 to 52 or 53 (except for year(s) involved in the
Julian-Gregorian transition).

The getFirstDayOfWeek() and getMinimalDaysInFirstWeek() values are
initialized using locale-dependent resources when constructing a
GregorianCalendar. The week determination is compatible with the ISO
8601 standard when getFirstDayOfWeek() is MONDAY and
getMinimalDaysInFirstWeek() is 4, which values are used in locales
where the standard is preferred. These values can explicitly be set by
calling setFirstDayOfWeek() and setMinimalDaysInFirstWeek().

A week year is in sync with a WEEK_OF_YEAR cycle. All weeks between
the first and last weeks (inclusive) have the same week year value.
Therefore, the first and last days of a week year may have different
calendar year values.

For example, January 1, 1998 is a Thursday. If getFirstDayOfWeek() is
MONDAY and getMinimalDaysInFirstWeek() is 4 (ISO 8601 standard
compatible setting), then week 1 of 1998 starts on December 29, 1997,
and ends on January 4, 1998. The week year is 1998 for the last three
days of calendar year 1997. If, however, getFirstDayOfWeek() is
SUNDAY, then week 1 of 1998 starts on January 4, 1998, and ends on
January 10, 1998; the first three days of 1998 then are part of week
53 of 1997 and their week year is 1997.

SimpleDateFormat Java dates

This is related to the use of yyyy vs YYYY. If you change your code to use yyyy it will work.

Here is the explanation of the difference between the two.

Week year

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