How to round DateTime of Joda library to the nearest X minutes?

The accepted answer doesn't correctly handle datetimes that have seconds or milliseconds set. For completeness, here's a version that does handle that correctly:

private DateTime roundDate(final DateTime dateTime, final int minutes) {
    if (minutes < 1 || 60 % minutes != 0) {
        throw new IllegalArgumentException("minutes must be a factor of 60");
    }

    final DateTime hour = dateTime.hourOfDay().roundFloorCopy();
    final long millisSinceHour = new Duration(hour, dateTime).getMillis();
    final int roundedMinutes = ((int)Math.round(
        millisSinceHour / 60000.0 / minutes)) * minutes;
    return hour.plusMinutes(roundedMinutes);
}

I once hacked this Method to do something like it. It's not optimized in any way, but it did what I wanted at the time. Never made it in any production environment, and I cannot tell you anything about performance.

@Test
     public void test() {
         System.out.println(roundDate(new DateTime().withMinuteOfHour(13)));
         System.out.println(roundDate(new DateTime().withMinuteOfHour(48)));
         System.out.println(roundDate(new DateTime().withMinuteOfHour(0)));
         System.out.println(roundDate(new DateTime().withMinuteOfHour(59)));
         System.out.println(roundDate(new DateTime().withMinuteOfHour(22)));
         System.out.println(roundDate(new DateTime().withMinuteOfHour(37)));
     }

    private DateTime roundDate(final DateTime dateTime) {
        final double minuteOfHour = dateTime.getMinuteOfHour();
        final double tenth = minuteOfHour / 10;
        final long round = Math.round(tenth);
        final int i = (int) (round * 10);

        if (i == 60) {
            return dateTime.plusHours(1).withMinuteOfHour(0);
        } else {
            return dateTime.withMinuteOfHour(i);
        }

    }

Using pure DateTime (Joda) Java Library:

DateTime dt = new DateTime(1385577373517L, DateTimeZone.UTC);
// Prints 2013-11-27T18:36:13.517Z
System.out.println(dt);

// Prints 2013-11-27T18:36:00.000Z (Floor rounded to a minute)
System.out.println(dt.minuteOfDay().roundFloorCopy());

// Prints 2013-11-27T18:30:00.000Z (Rounded to custom minute Window)
int windowMinutes = 10;
System.out.println(
    dt.withMinuteOfHour((dt.getMinuteOfHour() / windowMinutes) * windowMinutes)
        .minuteOfDay().roundFloorCopy()
    );

Here's another approach that uses arithmetic on Unix time for completeness:

(Implemented in Scala for clarity.)

import org.joda.time.{DateTime, Duration}

def roundDateTime(t: DateTime, d: Duration) = {
  t minus (t.getMillis - (t.getMillis.toDouble / d.getMillis).round * d.getMillis)
}

Example usage:

roundDateTime(new DateTime("2013-06-27T11:32:00"), Duration.standardMinutes(10))
// => 2013-06-27T11:30:00.000+02:00

roundDateTime(new DateTime("2013-06-27T11:37:00"), Duration.standardMinutes(10))
// => 2013-06-27T11:40:00.000+02:00