java.util.Date represents an instant in time, with no reference to a particular time zone or calendar system. It does hold both date and time though - it's basically a number of milliseconds since the Unix epoch.

Alternatively you can use java.util.Calendar which does know about both of those things.

Personally I would strongly recommend you use Joda Time which is a much richer date/time API. It allows you to express your data much more clearly, with types for "just dates", "just local times", "local date/time", "instant", "date/time with time zone" etc. Most of the types are also immutable, which is a huge benefit in terms of code clarity.

Use LocalDateTime#parse() (or ZonedDateTime#parse() if the string happens to contain a time zone part) to parse a String in a certain pattern into a LocalDateTime.

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

Then use LocalDateTime#format() (or ZonedDateTime#format()) to format a LocalDateTime into a String in a certain pattern.

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

Or, when you're not on Java 8 yet, use SimpleDateFormat#parse() to parse a String in a certain pattern into a Date.

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

Then use SimpleDateFormat#format() to format a Date into a String in a certain pattern.

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

See also:

• Java string to date conversion

Update: as per your failed attempt which you added to the question after this answer was posted; the patterns are case sensitive. Carefully read the java.text.SimpleDateFormat javadoc what the individual parts stands for. So stands for example M for months and m for minutes. Also, years exist of four digits yyyy, not five yyyyy. Look closer at the code snippets I posted here above.

Each one of the Date classes are for specific purposes:

• If you want to use your Date in an SQL/JDBC context, use the java.sql.Timestamp.

• java.util.Date is the old Java API, it is not thread safe, you can difficultly handle time zoning, and on the top of all, it is poorly designed: one simple uniformity is that months start from 1 while days start from 0.

• java.time.LocalDateTime is an immutable date-time object that represents a date-time, often viewed as year-month-day-hour-minute-second, which you need exactly.

• java.time.ZonedDateTime class stores all date and time fields, so you can use it to deal with values like: 27th January 1990 at 15:40.30.123123123 +02:00 in the Europe/Paris time-zone.

To do your task, the ZonedDateTime class handles conversion from the local time-line of LocalDateTime to the instant time-line of Instant (which models a single instantaneous point on the time-line). The difference between the two time-lines, represented by a ZoneOffset, is the offset from UTC/Greenwich.

To calculate duration and period: there is the java.time.Duration which is a time-based amount of time, such as '20.5 seconds', and java.time.Period, which is a date-based amount of time (like: 26 years, 2 months and 2 days).

To get max and min dates, you can use the Java 8 lambdas in something like:

Date maxDate = list.stream().map(yourInstance -> yourInstance.date).max(Date::compareTo).get();
Date minDate = list.stream().map(yourInstance -> yourInstance.date).min(Date::compareTo).get();