Joda time plusDays getting wrong date

I am using this code:

 DateTime(date)//Thu Aug 06 00:00:00 GMT+03:00 2020
                .plusDays(days) // 103
                .toDate()

And the result is Fri Dec 18 23:00:00 GMT+02:00 2020 instead of Dec 19.
With some dates it work well, with other the result date-1, I guess the problem is with number of days in month, but does plusDays() not consider it?

You could use java.time and it is supported in lower Android APIs, because there's API desugaring in Android now.

There's a zone-aware class (java.time.ZonedDateTime) and an offset-aware one (java.time.OffsetDateTime), but your example String just contains an offset from GMT / UTC. That's why I would use an OffsetDateTime that parses the exact moment in time and then adds a day.

Here's a simple example that defines a formatter which parses the given String and uses it for the output:

public static void main(String[] args) {
	// example String
	String date = "Fri Dec 18 23:00:00 GMT+02:00 2020";
	// create a formatter that is able to parse and output the String
	DateTimeFormatter dtf = DateTimeFormatter.ofPattern("EEE MMM dd HH:mm:ss OOOO uuuu",
														Locale.ENGLISH);
	// parse the String using the formatter defined above
	OffsetDateTime odt = OffsetDateTime.parse(date, dtf);
	System.out.println("OffsetDateTime parsed is\t" + odt.format(dtf));
	
	// add a day to the date part
	OffsetDateTime dayLater = odt.plusDays(1);
	System.out.println("Adding a day results in\t\t" + dayLater.format(dtf));
}

This outputs

OffsetDateTime parsed is	Fri Dec 18 23:00:00 GMT+02:00 2020
Adding a day results in		Sat Dec 19 23:00:00 GMT+02:00 2020

If you are interested in outputting dates only (no time part or offset), there's another handy thing in those classes, that is easy extraction of date- or time-part. You can do the following with an OffsetDateTime, for example:

// extract the part that only holds information about day of month, month of year and year
LocalDate dateOnly = odt.toLocalDate();
// print the default format (ISO standard)
System.out.println(dateOnly);
// or define and use a totally custom format
System.out.println(dateOnly.format(
						DateTimeFormatter.ofPattern("EEEE, 'the' dd. 'of' MMMM uuuu",
													Locale.ENGLISH)
					)
);

That would output

2020-12-18
Friday, the 18. of December 2020

In case you are dealing with a DatePicker datePicker, you can receive selected values by getYear(), getMonth() and getDayOfMonth(), then create a

LocalDate localDate = LocalDate.of(datePicker.getYear(),
                                   datePicker.getMonth(), 
                                   datePicker.getDayOfMonth());

and then simply add a day by localDate.plusDays(1);

> With some dates it work well, with other the result date-1, I guess
> the problem is with number of days in month, but does plusDays() not
> consider it?

It does consider it. The problem with the two date-time strings you have mentioned is that they belong to different Zone-Offset (the first one is with UTC+3 and the second one with UTC+2). Given below is how to do it with the same Zone-Offset.

import org.joda.time.DateTime;
import org.joda.time.DateTimeZone;
import org.joda.time.format.DateTimeFormat;
import org.joda.time.format.DateTimeFormatter;

public class Main {
	public static void main(String[] args) {
		DateTimeFormatter formatter = DateTimeFormat.forPattern("EEE MMM dd HH:mm:ss zZ yyyy");
		DateTime dateTime = DateTime.parse("Thu Aug 06 00:00:00 GMT+03:00 2020", formatter);
		System.out.println(dateTime);
		// With Zone-Offset of UTC+2
		System.out.println(dateTime.withZone(DateTimeZone.forOffsetHours(2)));

		// Add 103 days
		DateTime dateTimeAfter103Days = dateTime.plusDays(103);
		System.out.println(dateTimeAfter103Days);
		System.out.println(dateTime.withZone(DateTimeZone.forOffsetHours(2)));
	}
}

Output:

2020-08-05T21:00:00.000Z
2020-08-05T23:00:00.000+02:00
2020-11-16T21:00:00.000Z
2020-08-05T23:00:00.000+02:00

I recommend you use the modern java.time date-time API and the corresponding formatting API (package, java.time.format). Learn more about the modern date-time API from Trail: Date Time. If your Android API level is still not compliant with Java8, check https://stackoverflow.com/questions/38922754/how-to-use-threetenabp-in-android-project and Java 8+ APIs available through desugaring.

The following table shows an overview of modern date-time classes:

With modern date-time API:

import java.time.OffsetDateTime;
import java.time.format.DateTimeFormatter;

public class Main {
	public static void main(String[] args) {
		// Define formatter for your date-time string
		DateTimeFormatter formatter = DateTimeFormatter.ofPattern("EEE MMM dd HH:mm:ss O u");

		// Parse the given date0-time string into OffsetDateTime object
		OffsetDateTime dateTime = OffsetDateTime.parse("Thu Aug 06 00:00:00 GMT+03:00 2020", formatter);
		System.out.println(dateTime);

		// Add 103 days to the OffsetDateTime object
		OffsetDateTime dateTimeAfter103Days = dateTime.plusDays(103);
		System.out.println(dateTimeAfter103Days);
	}
}

Output:

2020-08-06T00:00+03:00
2020-11-17T00:00+03:00

Alternatively,

import java.time.LocalDateTime;
import java.time.OffsetDateTime;
import java.time.ZoneOffset;
import java.time.format.DateTimeFormatter;

public class Main {
	public static void main(String[] args) {
		// Define formatter for your date-time string
		DateTimeFormatter formatter = DateTimeFormatter.ofPattern("EEE MMM dd HH:mm:ss z yyyy");

		// If you do not need Zone Id or Zone Offset information, you can go for
		// LocalDateTime
		LocalDateTime dateTime = LocalDateTime.parse("Thu Aug 06 00:00:00 GMT+03:00 2020", formatter);
		System.out.println(dateTime);

		// You can convert LocalDateTime object into an OffsetDateTime by applying the
		// Zone-Offset e.g. the following line applies UTC+03:00 hours to LocalDateTime
		OffsetDateTime odt = dateTime.atOffset(ZoneOffset.ofHours(3));
		System.out.println(odt);

		// Add 103 days to the LocalDateTime object
		LocalDateTime dateTimeAfter103Days = dateTime.plusDays(103);
		System.out.println(dateTimeAfter103Days);

		odt = dateTimeAfter103Days.atOffset(ZoneOffset.ofHours(3));
		System.out.println(odt);
	}
}

Output:

2020-08-06T00:00
2020-08-06T00:00+03:00
2020-11-17T00:00
2020-11-17T00:00+03:00