Calendar`s getActualMinimum returns wrong value-CodePudding

I need to compare a certain date to the current date/time to make sure that it comes before the first day/hour/min/sec of the current month. In order to implement this feature, a configure a Calendar instance using its getActualMinimum method, however, today (Thursday, 19/01/2023 - 10:40:18 BRT 2023), it presented a behavior that I have never faced before. Consider the following code:

    Calendar cal = Calendar.getInstance();
    System.out.println("After instantiation:                  "   cal.getTime());
    
    cal.set(Calendar.DAY_OF_MONTH, cal.getActualMinimum(Calendar.DAY_OF_MONTH));
    System.out.println("After configuring the Day of Month:   "   cal.getTime());
    
    cal.set(Calendar.HOUR_OF_DAY, cal.getActualMinimum(Calendar.HOUR_OF_DAY));
    System.out.println("After configuring the Hour of day:    "   cal.getTime());
    
    cal.set(Calendar.MINUTE, cal.getActualMinimum(Calendar.MINUTE));
    System.out.println("After configuring the Minutes:        "   cal.getTime());
    
    cal.set(Calendar.SECOND, cal.getActualMinimum(Calendar.SECOND));
    System.out.println("After configuring the Seconds:        "   cal.getTime());
    
    cal.set(Calendar.MILLISECOND, cal.getActualMinimum(Calendar.MILLISECOND));
    System.out.println("After configuring the Millis:         "   cal.getTime());

The code above, in the moment that this post is being created, would print to the console:

After instantiation:                  Thu Jan 19 10:40:18 BRT 2023
After configuring the Day of Month:   Sun Jan 01 10:40:18 BRT 2023
After configuring the Hour of day:    Sat Dec 31 23:40:18 BRT 2022
After configuring the Minutes:        Sat Dec 31 23:00:18 BRT 2022
After configuring the Seconds:        Sat Dec 31 23:00:00 BRT 2022
After configuring the Millis:         Sat Dec 31 23:00:00 BRT 2022

Could someone explain why, after configuring the Hour of day, the value was set to 23 and not 00?

CodePudding user response：

Probably this is caused by the default TimeZone of your PC , try adding this after you declare the Calnedar object

cal.setTimeZone(TimeZone.getTimeZone("UTC"));

CodePudding user response：

java.time

The java.util Date-Time API and their formatting API, SimpleDateFormat are outdated and error-prone. It is recommended to stop using them completely and switch to the modern Date-Time API.

With java.time, the modern date-time API, you have specialized types for different purposes. A very common type is ZonedDateTime which contains the information of the timezone along with the date and time information.

Note: Unlike java.util date-time types, java.time types are immutable i.e. you always get a new instance on setting a new value; therefore, like a String, you need to assign the new value to the reference if you want the reference to point to the new value.

Demo:

import java.time.LocalDate;
import java.time.LocalTime;
import java.time.ZoneId;
import java.time.ZonedDateTime;
import java.time.format.DateTimeFormatter;
import java.time.temporal.ChronoField;
import java.time.temporal.TemporalAdjusters;
import java.util.Locale;

class Main {
    public static void main(String[] args) {
        // Replace ZoneId.systemDefault() with the applicable ZoneId e.g.
        // ZoneId.of("America/New_York")
        ZonedDateTime zdt = ZonedDateTime.now(ZoneId.systemDefault());
        System.out.println(zdt);

        zdt = zdt.with(TemporalAdjusters.firstDayOfMonth());
        System.out.println(zdt);

        zdt = zdt.withHour(LocalTime.MIN.getHour());
        System.out.println(zdt);

        zdt = zdt.withMinute(LocalTime.MIN.getMinute());
        System.out.println(zdt);

        zdt = zdt.withSecond(LocalTime.MIN.getSecond());
        System.out.println(zdt);

        zdt = zdt.with(ChronoField.MILLI_OF_SECOND, LocalTime.MIN.getLong(ChronoField.MILLI_OF_SECOND));
        DateTimeFormatter formatter = DateTimeFormatter.ofPattern("uuuu-MM-dd'T'HH:mm:ss.SSSXXX", Locale.ENGLISH);
        System.out.println(zdt.format(formatter));

        // In a single statement
        String output = ZonedDateTime.now(ZoneId.systemDefault())
                .with(TemporalAdjusters.firstDayOfMonth())
                .withHour(LocalTime.MIN.getHour())
                .withMinute(LocalTime.MIN.getMinute())
                .withSecond(LocalTime.MIN.getSecond())
                .with(ChronoField.MILLI_OF_SECOND, LocalTime.MIN.getLong(ChronoField.MILLI_OF_SECOND))
                .format(formatter);
        System.out.println(output);

        // There is a better way if all you want is day-1 with minimum time
        zdt = LocalDate.now(ZoneId.systemDefault())
                .with(TemporalAdjusters.firstDayOfMonth())
                .atStartOfDay()
                .atZone(ZoneId.systemDefault());
        System.out.println(zdt.format(formatter));
    }
}

Output from a sample run:

2023-01-19T16:50:43.811714Z[GMT]
2023-01-01T16:50:43.811714Z[GMT]
2023-01-01T00:50:43.811714Z[GMT]
2023-01-01T00:00:43.811714Z[GMT]
2023-01-01T00:00:00.811714Z[GMT]
2023-01-01T00:00:00.000Z
2023-01-01T00:00:00.000Z
2023-01-01T00:00:00.000Z

ONLINE DEMO

Learn more about the modern Date-Time API from Trail: Date Time.

In case you need a solution using the legacy API:

Calendar#getTime returns an instance of java.util.Date which is not a real date-time object; rather, it just contains the number of milliseconds from January 1, 1970, 00:00:00 GMT. The Date#toString applies the system's timezone to calculate the date-time and returns the same.

The way to get the date-time string with the desired timezone is by applying the timezone to the SimpleDateFormat and using it to format the instance of java.util.Date.

Demo:

import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Locale;
import java.util.TimeZone;

class Main {
    public static void main(String[] args) {
        SimpleDateFormat sdf = new SimpleDateFormat("EEE MMM dd HH:mm:ss.SSS z yyyy", Locale.ENGLISH);
        sdf.setTimeZone(TimeZone.getTimeZone("UTC"));

        Calendar cal = Calendar.getInstance();
        System.out.println("After instantiation:                  "   sdf.format(cal.getTime()));

        cal.set(Calendar.DAY_OF_MONTH, cal.getActualMinimum(Calendar.DAY_OF_MONTH));
        System.out.println("After configuring the Day of Month:   "   sdf.format(cal.getTime()));

        cal.set(Calendar.HOUR_OF_DAY, cal.getActualMinimum(Calendar.HOUR_OF_DAY));
        System.out.println("After configuring the Hour of day:    "   sdf.format(cal.getTime()));

        cal.set(Calendar.MINUTE, cal.getActualMinimum(Calendar.MINUTE));
        System.out.println("After configuring the Minutes:        "   sdf.format(cal.getTime()));

        cal.set(Calendar.SECOND, cal.getActualMinimum(Calendar.SECOND));
        System.out.println("After configuring the Seconds:        "   sdf.format(cal.getTime()));

        cal.set(Calendar.MILLISECOND, cal.getActualMinimum(Calendar.MILLISECOND));
        System.out.println("After configuring the Millis:         "   sdf.format(cal.getTime()));
    }
}

Output from a sample run:

After instantiation:                  Thu Jan 19 15:29:38.381 UTC 2023
After configuring the Day of Month:   Sun Jan 01 15:29:38.381 UTC 2023
After configuring the Hour of day:    Sun Jan 01 00:29:38.381 UTC 2023
After configuring the Minutes:        Sun Jan 01 00:00:38.381 UTC 2023
After configuring the Seconds:        Sun Jan 01 00:00:00.381 UTC 2023
After configuring the Millis:         Sun Jan 01 00:00:00.000 UTC 2023

ONLINE DEMO