How to get the first day of the current week and m

Question

I have the date of several events expressed in milliseconds[1], and I want to know which events are inside the current week and the current month, but I can't figure out how to obtain the first day (day/month/year) of the running week and convert it to milliseconds, the same for the first day of the month.

[1]Since January 1, 1970, 00:00:00 GMT

Answer 1

java.time

The java.time framework in Java 8 and later supplants the old java.util.Date/.Calendar classes. The old classes have proven to be troublesome, confusing, and flawed. Avoid them.

The java.time framework is inspired by the highly-successful Joda-Time library, defined by JSR 310, extended by the ThreeTen-Extra project, and explained in the Tutorial.

Instant

The Instant class represents a moment on the timeline in UTC.

The java.time framework has a resolution of nanoseconds, or 9 digits of a fractional second. Milliseconds is only 3 digits of a fractional second. Because millisecond resolution is common, java.time includes a handy factory method.

long millisecondsSinceEpoch = 1446959825213L;
Instant instant = Instant.ofEpochMilli ( millisecondsSinceEpoch );

millisecondsSinceEpoch: 1446959825213 is instant: 2015-11-08T05:17:05.213Z

ZonedDateTime

To consider current week and current month, we need to apply a particular time zone.

ZoneId zoneId = ZoneId.of ( "America/Montreal" );
ZonedDateTime zdt = ZonedDateTime.ofInstant ( instant , zoneId );

In zoneId: America/Montreal that is: 2015-11-08T00:17:05.213-05:00[America/Montreal]

Half-Open

In date-time work, we commonly use the Half-Open approach to defining a span of time. The beginning is inclusive while the ending in exclusive. Rather than try to determine the last split-second of the end of the week (or month), we get the first moment of the following week (or month). So a week runs from the first moment of Monday and goes up to but not including the first moment of the following Monday.

Let's the first day of the week, and last. The java.time framework includes a tool for that, the with method and the ChronoField enum.

By default, java.time uses the ISO 8601 standard. So Monday is the first day of the week (1) and Sunday is last (7).

ZonedDateTime firstOfWeek = zdt.with ( ChronoField.DAY_OF_WEEK , 1 ); // ISO 8601, Monday is first day of week.
ZonedDateTime firstOfNextWeek = firstOfWeek.plusWeeks ( 1 );

That week runs from: 2015-11-02T00:17:05.213-05:00[America/Montreal] to 2015-11-09T00:17:05.213-05:00[America/Montreal]

Oops! Look at the time-of-day on those values. We want the first moment of the day. The first moment of the day is not always 00:00:00.000 because of Daylight Saving Time (DST) or other anomalies. So we should let java.time make the adjustment on our behalf. To do that, we must go through the LocalDate class.

ZonedDateTime firstOfWeek = zdt.with ( ChronoField.DAY_OF_WEEK , 1 ); // ISO 8601, Monday is first day of week.
firstOfWeek = firstOfWeek.toLocalDate ().atStartOfDay ( zoneId );
ZonedDateTime firstOfNextWeek = firstOfWeek.plusWeeks ( 1 );

That week runs from: 2015-11-02T00:00-05:00[America/Montreal] to 2015-11-09T00:00-05:00[America/Montreal]

And same for the month.

ZonedDateTime firstOfMonth = zdt.with ( ChronoField.DAY_OF_MONTH , 1 );
firstOfMonth = firstOfMonth.toLocalDate ().atStartOfDay ( zoneId );
ZonedDateTime firstOfNextMonth = firstOfMonth.plusMonths ( 1 );

That month runs from: 2015-11-01T00:00-04:00[America/Montreal] to 2015-12-01T00:00-05:00[America/Montreal]

YearMonth

Another way to see if a pair of moments are in the same month is to check for the same YearMonth value.

For example, assuming thisZdt and thatZdt are both ZonedDateTime objects:

boolean inSameMonth = YearMonth.from( thisZdt ).equals( YearMonth.from( thatZdt ) ) ;

Milliseconds

I strongly recommend against doing your date-time work in milliseconds-from-epoch. That is indeed the way date-time classes tend to work internally, but we have the classes for a reason. Handling a count-from-epoch is clumsy as the values are not intelligible by humans so debugging and logging is difficult and error-prone. And, as we've already seen, different resolutions may be in play; old Java classes and Joda-Time library use milliseconds, while databases like Postgres use microseconds, and now java.time uses nanoseconds.

Would you handle text as bits, or do you let classes such as String, StringBuffer, and StringBuilder handle such details?

But if you insist, from a ZonedDateTime get an Instant, and from that get a milliseconds-count-from-epoch. But keep in mind this call can mean loss of data. Any microseconds or nanoseconds that you might have in your ZonedDateTime/Instant will be truncated (lost).

long millis = firstOfWeek.toInstant().toEpochMilli();  // Possible data loss.

Answer 2

Get First date of next month:-

SimpleDateFormat df = new SimpleDateFormat("MM-dd-yyyy");
String selectedDate="MM-dd-yyyy like 07-02-2018";
Date dt = df.parse(selectedDate);`enter code here`
calendar = Calendar.getInstance();
calendar.setTime(dt);
calendar.set(Calendar.DAY_OF_MONTH, calendar.getActualMaximum(Calendar.DAY_OF_MONTH) + 1);

String firstDate = df.format(calendar.getTime());
System.out.println("firstDateof next month ==>" + firstDate);

Answer 3

I have created some methods for this:

public static String catchLastDayOfCurrentWeek(String pattern) {
    Calendar cal = Calendar.getInstance();
    cal.set(Calendar.HOUR_OF_DAY, 0);
    cal.clear(Calendar.MINUTE);
    cal.clear(Calendar.SECOND);
    cal.clear(Calendar.MILLISECOND);

    cal.set(Calendar.DAY_OF_WEEK, cal.getFirstDayOfWeek());

    return calendarToString(cal, pattern);
}

public static String catchLastDayOfCurrentWeek(String pattern) {
    Calendar cal = Calendar.getInstance();
    cal.set(Calendar.HOUR_OF_DAY, 0);
    cal.clear(Calendar.MINUTE);
    cal.clear(Calendar.SECOND);
    cal.clear(Calendar.MILLISECOND);

    cal.set(Calendar.DAY_OF_WEEK, cal.getFirstDayOfWeek());

    cal.add(Calendar.WEEK_OF_YEAR, 1);
    cal.add(Calendar.MILLISECOND, -1);

    return calendarToString(cal, pattern);
}

public static String catchTheFirstDayOfThemonth(Integer month, pattern padrao) {
    Calendar cal = GregorianCalendar.getInstance();
    cal.setTime(new Date());
    cal.set(Calendar.MONTH, month);
    cal.set(Calendar.DAY_OF_MONTH, 1);

    return calendarToString(cal, pattern);
}

public static String catchTheLastDayOfThemonth(Integer month, String pattern) {
    Calendar cal = GregorianCalendar.getInstance();
    cal.setTime(new Date());
    cal.set(cal.get(Calendar.YEAR), month, cal.getActualMaximum(Calendar.DAY_OF_MONTH));

    return calendarToString(cal, pattern);
}

public static String calendarToString(Calendar calendar, String pattern) {
    if (calendar == null) {
        return "";
    }
    SimpleDateFormat format = new SimpleDateFormat(pattern, LocaleUtils.DEFAULT_LOCALE);
    return format.format(calendar.getTime());
}

You can see more here.

Answer 4

import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.GregorianCalendar;
import java.util.Scanner;

/**
This Program will display day for, 1st and last days in a given month and year

@author Manoj Kumar Dunna
Mail Id : [e-mail]
*/
public class DayOfWeek {
    public static void main(String[] args) {
        String strDate = null;
        int  year = 0, month = 0;
        Scanner sc = new Scanner(System.in);
        System.out.print("Enter YYYY/MM: ");
        strDate = sc.next();
        Calendar cal = new GregorianCalendar();
        String [] date = strDate.split("/");
        year = Integer.parseInt(date[0]);
        month = Integer.parseInt(date[1]);
        cal.set(year, month-1, 1);
        System.out.println(new SimpleDateFormat("EEEE").format(cal.getTime()));
        cal.add(Calendar.MONTH, 1);
        cal.add(Calendar.DAY_OF_YEAR, -1);
        System.out.println(new SimpleDateFormat("EEEE").format(cal.getTime()));
    }
}

Answer 5

Attention!

while (calendar.get(Calendar.DAY_OF_WEEK) > calendar.getFirstDayOfWeek()) {
    calendar.add(Calendar.DATE, -1); // Substract 1 day until first day of week.
}

is good idea, but there is some issue: For example, i'm from Ukraine and calendar.getFirstDayOfWeek() in my country is 2 (Monday). And today is 1 (Sunday). In this case calendar.add not called.

So, correct way is change ">" to "!=":

while (calendar.get(Calendar.DAY_OF_WEEK) != calendar.getFirstDayOfWeek()) {...

Answer 6

Simple Solution:
package com.util.calendarutil;

import java.text.DateFormat;
import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import java.util.GregorianCalendar;

public class CalUtil {
    public static void main(String args[]){
        DateFormat df = new SimpleDateFormat("dd/mm/yyyy");
        Date dt = null;
        try {
            dt = df.parse("23/01/2016");
        } catch (ParseException e) {
            System.out.println("Error");
        }

        Calendar cal = Calendar.getInstance();
        cal.setTime(dt);
        cal.set(Calendar.DAY_OF_WEEK, cal.getFirstDayOfWeek());
        Date startDate = cal.getTime();
        cal.add(Calendar.DATE, 6);
        Date endDate = cal.getTime();
        System.out.println("Start Date:"+startDate+"End Date:"+endDate);


    }

}