Java, get days between two dates

Question

In java, I want to get the number of days between two dates, excluding those two dates.

For example:

If first date = 11 November 2011 and the second date = 13 November 2011 then it should be 1.

This is the code I am using but doesn't work (secondDate and firstDate are Calendar objects):

long diff=secondDate.getTimeInMillis()-firstDate.getTimeInMillis();
float day_count=(float)diff / (24 * 60 * 60 * 1000);
daysCount.setText((int)day_count+"");                    

I even tried rounding the results but that didn't help.

How do I get the number of days between dates in java excluding the days themselves?

Answer 1

java.time

The java.time API, released with Java-8 in March 2014, supplanted the error-prone legacy date-time API. Since then, using this modern date-time API has been strongly recommended.

Solution using modern date-time API

Using Calendar#toInstant, convert your java.util.Calendar instances into java.time.Instant and then into java.time.ZonedDateTime instances and then use ChronoUnit.DAYS.between to get the number of days between them.

Demo:

import java.time.ZoneId;
import java.time.ZonedDateTime;
import java.time.temporal.ChronoUnit;
import java.util.Calendar;

public class Main {

    public static void main(String[] args) {
        // Sample start and end dates as java.util.Date
        Calendar startCal = Calendar.getInstance();
        startCal.set(2011, 10, 11); // 11 November 2011

        Calendar endCal = Calendar.getInstance();
        endCal.set(2011, 10, 13); // 13 November 2011

        // Convert the java.util.Calendar into java.time.ZonedDateTime
        // Replace ZoneId.systemDefault() with the applicable ZoneId
        ZonedDateTime startDateTime = startCal.toInstant().atZone(ZoneId.systemDefault());
        ZonedDateTime endDateTime = endCal.toInstant().atZone(ZoneId.systemDefault());

        // The end date is excluded by default. Subtract 1 to exclude the start date
        long days = ChronoUnit.DAYS.between(startDateTime, endDateTime) - 1;
        System.out.println(days);
    }
}

Output:

1

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

Answer 2

Get Days between java.util.Dates, ignoring daylight savings time

Quick and dirty hack:

public int get_days_between_dates(Date date1, Date date2)
{       
    //if date2 is more in the future than date1 then the result will be negative
    //if date1 is more in the future than date2 then the result will be positive.

    return (int)((date2.getTime() - date1.getTime()) / (1000*60*60*24l));
}

This function will work 99.99% of the time, except when it surprises you later on in the edge cases during leap-seconds, daylight savings, timezone changes leap years and the like. If you are OK with the calculation being off by 1 (or 2) hours once in a while, this will suffice.

Get Days between Dates taking into account leapseconds, daylight savings, timezones, etc

If you are asking this question you need to slap yourself. What does it mean for two dates to be at least 1 day apart? It's very confusing. What if one Date is midnight in one timezone, and the other date is 1AM in another timezone? Depending on how you interpret it, the answer is both 1 and 0.

You think you can just force the dates you pass into the above function as Universal time format; that will fix some of your problems. But then you just relocate the problem into how you convert your local time to a universal time. The logical conversion from your timezone to universal time may not be what is intuitive. In some cases you will get a day difference when the dates passed in are obviously two days apart.

And you think you can deal with that? There are some simplistic calendar systems in the world which are constantly changing depending on the harvest season and installed political rulers. If you want to convert their time to UTC, java.util.Date is going to fail you at the worst moment.

If you need to calculate the days between dates and it is critical that everything come out right, you need to get an external library called Joda Time: (They have taken care of all the details for you, so you can stay blissfully unaware of them): http://joda-time.sourceforge.net/index.html

Answer 3

I have two suggestions:

1. Make sure your float day_count is calculated correctly

   float day_count = ((float)diff) / (24f * 60f * 60f * 1000f);

2. If it's rounding error, try using floor method

   daysCount.setText("" + (int)Math.floor(day_count));

Answer 4

I've just tested on SDK 8 (Android 2.2) the following code snippet:

Calendar date1 = Calendar.getInstance();
Calendar date2 = Calendar.getInstance();

date1.clear();
date1.set(
   datePicker1.getYear(),
   datePicker1.getMonth(),
   datePicker1.getDayOfMonth());
date2.clear();
date2.set(
   datePicker2.getYear(),
   datePicker2.getMonth(),
   datePicker2.getDayOfMonth());

long diff = date2.getTimeInMillis() - date1.getTimeInMillis();

float dayCount = (float) diff / (24 * 60 * 60 * 1000);

textView.setText(Long.toString(diff) + " " + (int) dayCount);

it works perfectly and in both cases (Nov 10,2011 - Nov 8,2011) and (Nov 13,2011 - Nov 11,2011) gives dayCount = 2.0

Answer 5

If you are only going to be dealing with dates between the years 1900 and 2100, there is a simple calculation which will give you the number of days since 1900:

public static int daysSince1900(Date date) {
    Calendar c = new GregorianCalendar();
    c.setTime(date);

    int year = c.get(Calendar.YEAR);
    if (year < 1900 || year > 2099) {
        throw new IllegalArgumentException("daysSince1900 - Date must be between 1900 and 2099");
    }
    year -= 1900;
    int month = c.get(Calendar.MONTH) + 1;
    int days = c.get(Calendar.DAY_OF_MONTH);

    if (month < 3) {
        month += 12;
        year--;
    }
    int yearDays = (int) (year * 365.25);
    int monthDays = (int) ((month + 1) * 30.61);

    return (yearDays + monthDays + days - 63);
}

Thus, to get the difference in days between two dates, you calculate their days since 1900 and calc the difference. Our daysBetween method looks like this:

public static Integer getDaysBetween(Date date1, Date date2) {
    if (date1 == null || date2 == null) {
        return null;
    }

    int days1 = daysSince1900(date1);
    int days2 = daysSince1900(date2);

    if (days1 < days2) {
        return days2 - days1;
    } else {
        return days1 - days2;
    }
}

In your case you would need to subtract an extra day (if the days are not equal).

And don't ask me where this calculation came from because we've used it since the early '90s.

Answer 6

1. Don't use floats for integer calculations.

2. Are you sure your dates are days? The precision of the Date type is milliseconds. So the first thing you need to do is round the date to something which doesn't have hours. Example: It's just one hour from 23:30 2011-11-01 to 00:30 2011-11-02 but the two dates are on different days.