其他答案是正确的，基本上你在你的图案中有错误数量的“y”字符。

时区

还有一个问题，你没有提到时区。如果你指的是UTC，那么你应该这么说。如果不是，说明答案不完整。如果您只需要日期部分而不需要时间，那么没有问题。但是如果您要做的进一步工作可能涉及到时间，那么您应该指定一个时区。

乔达时间

下面是相同类型的代码，但是使用了第三方开源的Joda-Time 2.3库

// © 2013 Basil Bourque. This source code may be used freely forever by anyone taking full responsibility for doing so.

String date_s = "2011-01-18 00:00:00.0";

org.joda.time.format.DateTimeFormatter formatter = org.joda.time.format.DateTimeFormat.forPattern( "yyyy-MM-dd' 'HH:mm:ss.SSS" );
// By the way, if your date-time string conformed strictly to ISO 8601 including a 'T' rather than a SPACE ' ', you could
// use a formatter built into Joda-Time rather than specify your own: ISODateTimeFormat.dateHourMinuteSecondFraction().
// Like this:
//org.joda.time.DateTime dateTimeInUTC = org.joda.time.format.ISODateTimeFormat.dateHourMinuteSecondFraction().withZoneUTC().parseDateTime( date_s );

// Assuming the date-time string was meant to be in UTC (no time zone offset).
org.joda.time.DateTime dateTimeInUTC = formatter.withZoneUTC().parseDateTime( date_s );
System.out.println( "dateTimeInUTC: " + dateTimeInUTC );
System.out.println( "dateTimeInUTC (date only): " + org.joda.time.format.ISODateTimeFormat.date().print( dateTimeInUTC ) );
System.out.println( "" ); // blank line.

// Assuming the date-time string was meant to be in Kolkata time zone (formerly known as Calcutta). Offset is +5:30 from UTC (note the half-hour).
org.joda.time.DateTimeZone kolkataTimeZone = org.joda.time.DateTimeZone.forID( "Asia/Kolkata" );
org.joda.time.DateTime dateTimeInKolkata = formatter.withZone( kolkataTimeZone ).parseDateTime( date_s );
System.out.println( "dateTimeInKolkata: " + dateTimeInKolkata );
System.out.println( "dateTimeInKolkata (date only): " + org.joda.time.format.ISODateTimeFormat.date().print( dateTimeInKolkata ) );
// This date-time in Kolkata is a different point in the time line of the Universe than the dateTimeInUTC instance created above. The date is even different.
System.out.println( "dateTimeInKolkata adjusted to UTC: " + dateTimeInKolkata.toDateTime( org.joda.time.DateTimeZone.UTC ) );

运行时……

dateTimeInUTC: 2011-01-18T00:00:00.000Z
dateTimeInUTC (date only): 2011-01-18

dateTimeInKolkata: 2011-01-18T00:00:00.000+05:30
dateTimeInKolkata (date only): 2011-01-18
dateTimeInKolkata adjusted to UTC: 2011-01-17T18:30:00.000Z

从格式中删除一个y:

SimpleDateFormat dt1 = new SimpleDateFormat("yyyyy-mm-dd");

它应该是:

SimpleDateFormat dt1 = new SimpleDateFormat("yyyy-mm-dd");

答案当然是创建一个SimpleDateFormat对象，并使用它来解析“字符串到日期”并将“日期到字符串”格式化。如果您已经尝试了SimpleDateFormat，它没有工作，那么请显示您的代码和您可能收到的任何错误。

附录:String格式中的“mm”与“mm”不一样。用MM表示月，用MM表示分钟。另外，yyyyy和yyyy也不一样。例如,:

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;

public class FormateDate {

    public static void main(String[] args) throws ParseException {
        String date_s = "2011-01-18 00:00:00.0";

        // *** note that it's "yyyy-MM-dd hh:mm:ss" not "yyyy-mm-dd hh:mm:ss"  
        SimpleDateFormat dt = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss");
        Date date = dt.parse(date_s);

        // *** same for the format String below
        SimpleDateFormat dt1 = new SimpleDateFormat("yyyy-MM-dd");
        System.out.println(dt1.format(date));
    }

}

[编辑以包括BalusC的更正] SimpleDateFormat类应该做到这一点:

String pattern = "yyyy-MM-dd HH:mm:ss.S";
SimpleDateFormat format = new SimpleDateFormat(pattern);
try {
  Date date = format.parse("2011-01-18 00:00:00.0");
  System.out.println(date);
} catch (ParseException e) {
  e.printStackTrace();
}

使用LocalDateTime#parse()(如果字符串碰巧包含时区部分，则使用ZonedDateTime#parse())将某个模式下的string解析为LocalDateTime。

String oldstring = "2011-01-18 00:00:00.0";
LocalDateTime datetime = LocalDateTime.parse(oldstring, DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss.S"));

然后使用localdatetime# format()(或zoneddatetime# format())将LocalDateTime格式化为特定模式的字符串。

String newstring = datetime.format(DateTimeFormatter.ofPattern("yyyy-MM-dd"));
System.out.println(newstring); // 2011-01-18

或者，如果您还没有使用Java 8，可以使用SimpleDateFormat#parse()将特定模式下的String解析为Date。

String oldstring = "2011-01-18 00:00:00.0";
Date date = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss.S").parse(oldstring);

然后使用SimpleDateFormat#format()将日期格式化为特定模式的字符串。

String newstring = new SimpleDateFormat("yyyy-MM-dd").format(date);
System.out.println(newstring); // 2011-01-18

参见:

Java字符串到日期的转换

更新:根据你的失败的尝试，你添加到这个问题后的答案张贴;模式是区分大小写的。仔细阅读java.text.SimpleDateFormat javadoc中各个部分代表什么。举个例子，M代表月，M代表分钟。此外，年份是四位数yyyy，而不是五位数yyyyy。仔细看看我上面发布的代码片段。

/**
 * Method will take Date in "MMMM, dd yyyy HH:mm:s" format and return time difference like added: 3 min ago
 *
 * @param date : date in "MMMM, dd yyyy HH:mm:s" format
 * @return : time difference
 */
private String getDurationTimeStamp(String date) {
    String timeDifference = "";

    //date formatter as per the coder need
    SimpleDateFormat sdf = new SimpleDateFormat("MMMM, dd yyyy HH:mm:s");
    TimeZone timeZone = TimeZone.getTimeZone("EST");
    sdf.setTimeZone(timeZone);
    Date startDate = null;
    try {
        startDate = sdf.parse(date);
    } catch (ParseException e) {
        MyLog.printStack(e);
    }

    //end date will be the current system time to calculate the lapse time difference
    Date endDate = new Date();

    //get the time difference in milliseconds
    long duration = endDate.getTime() - startDate.getTime();

    long diffInSeconds = TimeUnit.MILLISECONDS.toSeconds(duration);
    long diffInMinutes = TimeUnit.MILLISECONDS.toMinutes(duration);
    long diffInHours = TimeUnit.MILLISECONDS.toHours(duration);
    long diffInDays = TimeUnit.MILLISECONDS.toDays(duration);

    if (diffInDays >= 365) {
        int year = (int) (diffInDays / 365);
        timeDifference = year + mContext.getString(R.string.year_ago);
    } else if (diffInDays >= 30) {
        int month = (int) (diffInDays / 30);
        timeDifference = month + mContext.getString(R.string.month_ago);
    }
    //if days are not enough to create year then get the days
    else if (diffInDays >= 1) {
        timeDifference = diffInDays + mContext.getString(R.string.day_ago);
    }
    //if days value<1 then get the hours
    else if (diffInHours >= 1) {
        timeDifference = diffInHours + mContext.getString(R.string.hour_ago);
    }
    //if hours value<1 then get the minutes
    else if (diffInMinutes >= 1) {
        timeDifference = diffInMinutes + mContext.getString(R.string.min_ago);
    }
    //if minutes value<1 then get the seconds
    else if (diffInSeconds >= 1) {
        timeDifference = diffInSeconds + mContext.getString(R.string.sec_ago);
    } else if (timeDifference.isEmpty()) {
        timeDifference = mContext.getString(R.string.now);
    }

    return mContext.getString(R.string.added) + " " + timeDifference;
}