How to round time to the nearest quarter hour in java?

One more alternate approach using java Instant api.

Instant instant =  Instant.now();
int intervalInMinutes = 10;
instant.truncatedTo(ChronoUnit.MINUTES).minus(instant.atZone(ZoneId.of("UTC")).getMinute() % (1* intervalInMinutes),ChronoUnit.MINUTES);

If you need to round down time to the nearest arbitrary level provided as Duration:

static long truncateTo(long timeEpochMillis, Duration d) {
    long x = timeEpochMillis / d.toMillis();
    return x * d.toMillis();
}

java.time

I recommend you do it using the the modern date-time API^*:

import java.time.LocalDate;
import java.time.LocalTime;
import java.time.ZoneId;
import java.time.ZonedDateTime;
import java.time.temporal.ChronoUnit;
import java.util.stream.Stream;

public class Main {
    public static void main(String[] args) {
        
        // Change it to the applicable ZoneId e.g. ZoneId.of("Asia/Kolkata")
        ZoneId zoneId = ZoneId.systemDefault();
        
        Stream.of(
                        "10:00", 
                        "10:05", 
                        "10:10", 
                        "10:15", 
                        "10:20", 
                        "10:25", 
                        "10:30"
            ).forEach(t -> System.out.println(roundToNearestQuarter(t, zoneId)));
    }

    static ZonedDateTime roundToNearestQuarter(String strTime, ZoneId zoneId) {
        LocalTime time = LocalTime.parse(strTime);
        return LocalDate.now()
                        .atTime(time)
                        .atZone(zoneId)
                        .truncatedTo(ChronoUnit.HOURS)
                        .plusMinutes(15 * Math.round(time.getMinute() / 15.0));
    }
}

Output:

2021-04-02T10:00+01:00[Europe/London]
2021-04-02T10:00+01:00[Europe/London]
2021-04-02T10:15+01:00[Europe/London]
2021-04-02T10:15+01:00[Europe/London]
2021-04-02T10:15+01:00[Europe/London]
2021-04-02T10:30+01:00[Europe/London]
2021-04-02T10:30+01:00[Europe/London]

In case you are looking for just time, use ZonedDateTime#toLocalTime to get the LocalTime from the obtained ZonedDateTime.

Learn more about the modern date-time API from Trail: Date 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^{* The java.util date-time API and their formatting API, SimpleDateFormat are outdated and error-prone.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. For any reason, if you have to stick to Java 6 or Java 7, you can use ThreeTen-Backport which backports most of the java.time functionality to Java 6 & 7. If you are working for an Android project and your Android API level is still not compliant with Java-8, check Java 8+ APIs available through desugaring and How to use ThreeTenABP in Android Project.}

public static Date getCurrentDateTimeWithQuarterRounding() {
    final Calendar calendar = new GregorianCalendar();
    calendar.setTime(new Date());
    calendar.set(Calendar.MILLISECOND, 0);
    calendar.set(Calendar.SECOND, 0);
    final int minutes = calendar.get(Calendar.MINUTE);

    if (minutes < 15) {
        calendar.set(Calendar.MINUTE, 0);
    } else if (minutes >= 45) {
        calendar.set(Calendar.MINUTE, 45);
    } else if (minutes < 30) {
        calendar.set(Calendar.MINUTE, 15);
    } else {
        calendar.set(Calendar.MINUTE, 30);
    }

    return calendar.getTime();
}

Use the following functions to get the minutes rounded to last quarter getRecentQuater():Date, getSysDate_LastQuarterMins("dd.MM.yyyy HH:mm:ss"):String: ^{Converting LocalDateTime to Date}

public static Date getRecentQuater() {
    LocalDateTime time = LocalDateTime.now();
    LocalDateTime lastQuarter = time.truncatedTo(ChronoUnit.HOURS).plusMinutes(getLastQuarterValue(time.getMinute()));
    System.out.println("lastQuarter LocalDateTime: " + lastQuarter);

    Date date = Date.from(lastQuarter.atZone(ZoneId.systemDefault()).toInstant());
    System.out.println("lastQuarter Date: " + lastQuarter);
    return date;
}
public static String getSysDate_LastQuarterMins(String dateFormat) {
    Date date = getRecentQuater();
    SimpleDateFormat ft = new SimpleDateFormat (dateFormat);
    String sysDate_RoundMin = ft.format(date);
    System.out.println("getSysDate_LastQuarterMins() LocalDateTime : "+sysDate_RoundMin);
    return sysDate_RoundMin;
}

getSysDate_LastQuarterMins() : Mon Jan 20 17:30:00 CET 2020

public static Date getSysDate_LastQuarterMins() {
    Calendar cal = Calendar.getInstance();
    cal.setTime( new Date(System.currentTimeMillis()) );

    int min = cal.get(Calendar.MINUTE);

    cal.set(Calendar.MINUTE, getLastQuarterValue(min));
    cal.set(Calendar.SECOND, 00);
    Date lastQuarter = cal.getTime();
    System.out.println("getSysDate_LastQuarterMins() Calendar : "+lastQuarter);
    return lastQuarter;
}

You can find the LastQuarter Value Round value from the follwing fucntions, provided with some outputs on function call diaplayLastQuarter_RoundValue(min):

Min: 10, LastQuarter:  0, Round: 15
Min: 24, LastQuarter: 15, Round: 30
Min: 36, LastQuarter: 30, Round: 30
Min: 37, LastQuarter: 30, Round: 30
Min: 38, LastQuarter: 30, Round: 45
Min: 39, LastQuarter: 30, Round: 45
Min: 44, LastQuarter: 30, Round: 45
Min: 57, LastQuarter: 45, Round: 00 [57, 07:45:00, 08:00:00]

public static void diaplayLastQuarter_RoundValue(int minutes) {
    System.out.format("Min: %2d, LastQuarter: %2d, Round: %2d\n",
            minutes, getLastQuarterValue(minutes), getRoundValue(minutes));
}
public static int getLastQuarterValue(int minutes) {
    int min = 15 * (minutes / 15);
    //System.out.println("Min: "+minutes+", getLastQuarterValue : "+ min);
    return min;
}
public static int getRoundValue(int minutes) {
    getLastQuarterValue(minutes);
    int minRound = (int) (Math.round(minutes / 15.0) * 15.0);
    //System.out.println("Min: "+minutes+", getRoundValue : "+minRound);
    return minRound;
}

If someone is interested to get the nearest (up or down) five or fifteen interval, I made a function using module that does the job.

public LocalTime roundToTheNearestInterval(LocalTime original, Integer measurementInterval) {
        LocalTime nearest;
        int mod;
        switch (measurementInterval) {
            case 5:
                mod = original.getMinute() % 5;
                nearest = mod >= 3 ?
                        original.truncatedTo(ChronoUnit.HOURS)
                                .plusMinutes((long) 5 * (original.getMinute() / 5) + 5) :
                        original.truncatedTo(ChronoUnit.HOURS)
                                .plusMinutes((long) 5 * (original.getMinute() / 5));
                break;
            case 15:
                mod = original.getMinute() % 15;
                nearest = mod >= 8 ?
                        original.truncatedTo(ChronoUnit.HOURS)
                                .plusMinutes((long) 15 * (original.getMinute() / 15) + 15) :
                        original.truncatedTo(ChronoUnit.HOURS)
                                .plusMinutes((long) 15 * (original.getMinute() / 15));
                break;
            default:
                nearest = original;
        }
        return nearest;
    }

You can try it with this unit test

 @Test
    void roundToTheNearestInterval() {
        //given
        LocalTime originalTime1 = LocalTime.of(6, 31, 15);
        LocalTime originalTime2 = LocalTime.of(19, 13, 42);
        LocalTime originalTime3 = LocalTime.of(6, 37, 11);
        LocalTime originalTime4 = LocalTime.of(19, 40, 34);
        Integer measurementInterval_5min = 5;
        Integer measurementInterval_15min = 15;

        MyService myService = new MyService();

        //when
        LocalTime rounded1_5min = myService.roundToTheNearestInterval(originalTime1, measurementInterval_5min);
        LocalTime rounded2_5min = myService.roundToTheNearestInterval(originalTime2, measurementInterval_5min);

        LocalTime rounded1_15min = myService.roundToTheNearestInterval(originalTime3, measurementInterval_15min);
        LocalTime rounded2_15min = myService.roundToTheNearestInterval(originalTime4, measurementInterval_15min);

        //then
        assertEquals(LocalTime.of(6, 30, 0), rounded1_5min);
        assertEquals(LocalTime.of(19, 15, 0), rounded2_5min);
        assertEquals(LocalTime.of(6, 30, 0), rounded1_15min);
        assertEquals(LocalTime.of(19, 45, 0), rounded2_15min);

    }