Java - calculate difference between two ZonedDateTime in days

import java.time.ZoneId;

import java.time.ZonedDateTime;

import java.util.concurrent.TimeUnit;

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public class ZonedDateTimeDifferenceCalculator {

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    public static long calculateDifferenceInDays(ZonedDateTime from, ZonedDateTime now) {

        long secondsDiff = now.toEpochSecond() - from.toEpochSecond();

        return TimeUnit.DAYS.convert(secondsDiff, TimeUnit.SECONDS);

    }

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    // usage example:

    public static void main(String[] args) {

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        ZonedDateTime from = ZonedDateTime.of(2019, 10,  3, 16, 20, 0, 0, ZoneId.of("UTC"));

        ZonedDateTime now  = ZonedDateTime.of(2019, 10, 12, 16, 20, 0, 0, ZoneId.of("UTC"));

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        long differenceDays = calculateDifferenceInDays(from, now);

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        System.out.println("Diff in days: ");

        System.out.println(differenceDays); // 9

    }

}

import org.assertj.core.api.Assertions;

import org.junit.jupiter.api.Test;

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import java.time.ZoneId;

import java.time.ZonedDateTime;

​

class ZonedDateTimeDifferenceCalculatorTest {

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    @Test

    void shouldCalculateDifferenceInDays() {

        // given

        ZonedDateTime from = ZonedDateTime.of(2019, 10,  3, 16, 20, 0, 0, ZoneId.of("UTC"));

        ZonedDateTime now  = ZonedDateTime.of(2019, 10, 12, 16, 20, 0, 0, ZoneId.of("UTC"));

​

        // when

        long actual = ZonedDateTimeDifferenceCalculator.calculateDifferenceInDays(from, now);

​

        // then

        Assertions.assertThat(actual).isEqualTo(9);

    }

}