public class SystemDateTest {

    String stringDate;

    public static void main(String[] args) {
        SystemDateTest systemDateTest = new SystemDateTest();
        // format date into String
        SimpleDateFormat simpleDateFormat = new SimpleDateFormat("dd-MM-yyyy hh:mm:ss");
        systemDateTest.setStringDate(simpleDateFormat.format(systemDateTest.getDate()));
        System.out.println(systemDateTest.getStringDate());
    }

    public Date getDate() {
        return new Date();
    }

    public String getStringDate() {
        return stringDate;
    }

    public void setStringDate(String stringDate) {
        this.stringDate = stringDate;
    }
}

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");

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;
}

假设您想将2019-12-20 10:50 AM GMT+6:00更改为2019-12-20 10:50 AM 首先，你们要理解日期格式首先，日期格式是 yyyy-MM-dd hh:mm a zzz和第二个日期格式将是yyyy-MM-dd hh:mm a

只要从这个函数返回一个字符串。

public String convertToOnlyDate(String currentDate) {
    SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd hh:mm a ");
    Date date;
    String dateString = "";
    try {
        date = dateFormat.parse(currentDate);
        System.out.println(date.toString()); 

        dateString = dateFormat.format(date);
    } catch (ParseException e) {
        e.printStackTrace();
    }
    return dateString;
}

这个函数将返回您想要的答案。如果你想自定义更多，只需从日期格式中添加或删除组件。

你也可以使用substring()

String date_s = "2011-01-18 00:00:00.0";
date_s.substring(0,10);

如果你想在日期前留出空格，那就用吧

String date_s = " 2011-01-18 00:00:00.0";
date_s.substring(1,11);