The user could change the selected month & year by clicking the next/previous month buttons, selecting a month from the month popup menu, or selecting a different year (using the year spinner): Edit: added example with adapted versions of JDatePicker classes Background: Most date-and-time-libraries (java.util.Calendar in Java, see also .Net-DateTime or Date in JavaScript or DateTime in Perl) are based on the concept of a universal all-purpose unique temporal type (in German there is the poetic expression "eierlegende Wollmilchsau"). In this design there cannot be an unsupported field. この変換では、yearおよびmonth_of_yearフィールドを抽出します。 この抽出が許可されるのは、時間的オブジェクトにISOの暦が含まれているか、そのオブジェクトを LocalDate に変換できる場合にかぎられます。 how to set DATE using constructor? public Date(int day, int month, int year) { this.day=day; // set day to 1-31 this.month=month; // set month to 1-12 this.year=year; // set year to 1900-9999 } if i use To answer your original question: Just like you are subtracting days that already belongs to year from diff to calculate months, you need to subtract . sum of days used on year and month to calculate weeks; sum of days which already belong to year, month, week if you want to calculate days; so your code can look like The Java Date and Time API’s WeekFields class represents the week-based year and its components, including the week number, day of the week, and week-based year. When the first day of the week is Sunday, the numbering of days of the week is from 1 to 7, with 1 being Sunday and 7 being Saturday. In this short tutorial, we’ll learn how to extract the year, month, and day from a given Date in Java. We’ll discuss how to extract these values using the legacy java.util.Date class, and also by using the new date-time library of Java 8. In Java 8, a whole new date and time library was introduced for a number of good reasons. Using a GregorianCalendar, the DAY_OF_MONTH field cannot be 31 in the month April. DAY_OF_MONTH is set to the closest possible value, 30. The YEAR field maintains the value of 1999 because it is a larger field than MONTH. Example: Consider a GregorianCalendar originally set to Sunday June 6, 1999. WA8lEMY.