Class Date
Prior to JDK 1.1, the class Date had two additional functions. It allowed the interpretation of dates as year, month, day, hour, minute, and second values. It also allowed the formatting and parsing of date strings. Unfortunately, the API for these functions was not amenable to internationalization. As of JDK 1.1, the Calendar class should be used to convert between dates and time fields and the DateFormat class should be used to format and parse date strings. The corresponding methods in Date are deprecated.
Although the Date class is intended to reflect coordinated universal time (UTC), it may not do so exactly, depending on the host environment of the Java Virtual Machine. Nearly all modern operating systems assume that 1 day = 24 × 60 × 60 = 86400 seconds in all cases. In UTC, however, about once every year or two there is an extra second, called a «leap second.» The leap second is always added as the last second of the day, and always on December 31 or June 30. For example, the last minute of the year 1995 was 61 seconds long, thanks to an added leap second. Most computer clocks are not accurate enough to be able to reflect the leap-second distinction.
Some computer standards are defined in terms of Greenwich mean time (GMT), which is equivalent to universal time (UT). GMT is the «civil» name for the standard; UT is the «scientific» name for the same standard. The distinction between UTC and UT is that UTC is based on an atomic clock and UT is based on astronomical observations, which for all practical purposes is an invisibly fine hair to split. Because the earth’s rotation is not uniform (it slows down and speeds up in complicated ways), UT does not always flow uniformly. Leap seconds are introduced as needed into UTC so as to keep UTC within 0.9 seconds of UT1, which is a version of UT with certain corrections applied. There are other time and date systems as well; for example, the time scale used by the satellite-based global positioning system (GPS) is synchronized to UTC but is not adjusted for leap seconds. An interesting source of further information is the United States Naval Observatory (USNO):
and the material regarding «Systems of Time» at:
which has descriptions of various different time systems including UT, UT1, and UTC.
- A year y is represented by the integer y — 1900 .
- A month is represented by an integer from 0 to 11; 0 is January, 1 is February, and so forth; thus 11 is December.
- A date (day of month) is represented by an integer from 1 to 31 in the usual manner.
- An hour is represented by an integer from 0 to 23. Thus, the hour from midnight to 1 a.m. is hour 0, and the hour from noon to 1 p.m. is hour 12.
- A minute is represented by an integer from 0 to 59 in the usual manner.
- A second is represented by an integer from 0 to 61; the values 60 and 61 occur only for leap seconds and even then only in Java implementations that actually track leap seconds correctly. Because of the manner in which leap seconds are currently introduced, it is extremely unlikely that two leap seconds will occur in the same minute, but this specification follows the date and time conventions for ISO C.
In all cases, arguments given to methods for these purposes need not fall within the indicated ranges; for example, a date may be specified as January 32 and is interpreted as meaning February 1.
Constructor Summary
Allocates a Date object and initializes it so that it represents the time at which it was allocated, measured to the nearest millisecond.
As of JDK version 1.1, replaced by Calendar.set(year + 1900, month, date) or GregorianCalendar(year + 1900, month, date) .
As of JDK version 1.1, replaced by Calendar.set(year + 1900, month, date, hrs, min) or GregorianCalendar(year + 1900, month, date, hrs, min) .
As of JDK version 1.1, replaced by Calendar.set(year + 1900, month, date, hrs, min, sec) or GregorianCalendar(year + 1900, month, date, hrs, min, sec) .
Allocates a Date object and initializes it to represent the specified number of milliseconds since the standard base time known as «the epoch», namely January 1, 1970, 00:00:00 GMT.
Package java.time
The main API for dates, times, instants, and durations.
The classes defined here represent the principle date-time concepts, including instants, durations, dates, times, time-zones and periods. They are based on the ISO calendar system, which is the de facto world calendar following the proleptic Gregorian rules. All the classes are immutable and thread-safe.
Each date time instance is composed of fields that are conveniently made available by the APIs. For lower level access to the fields refer to the java.time.temporal package. Each class includes support for printing and parsing all manner of dates and times. Refer to the java.time.format package for customization options.
The java.time.chrono package contains the calendar neutral API ChronoLocalDate , ChronoLocalDateTime , ChronoZonedDateTime and Era . This is intended for use by applications that need to use localized calendars. It is recommended that applications use the ISO-8601 date and time classes from this package across system boundaries, such as to the database or across the network. The calendar neutral API should be reserved for interactions with users.
Dates and Times
Instant is essentially a numeric timestamp. The current Instant can be retrieved from a Clock . This is useful for logging and persistence of a point in time and has in the past been associated with storing the result from System.currentTimeMillis() .
LocalDate stores a date without a time. This stores a date like ‘2010-12-03’ and could be used to store a birthday.
LocalTime stores a time without a date. This stores a time like ’11:30′ and could be used to store an opening or closing time.
LocalDateTime stores a date and time. This stores a date-time like ‘2010-12-03T11:30’.
ZonedDateTime stores a date and time with a time-zone. This is useful if you want to perform accurate calculations of dates and times taking into account the ZoneId , such as ‘Europe/Paris’. Where possible, it is recommended to use a simpler class without a time-zone. The widespread use of time-zones tends to add considerable complexity to an application.
Duration and Period
Beyond dates and times, the API also allows the storage of periods and durations of time. A Duration is a simple measure of time along the time-line in nanoseconds. A Period expresses an amount of time in units meaningful to humans, such as years or days.
Additional value types
Month stores a month on its own. This stores a single month-of-year in isolation, such as ‘DECEMBER’.
DayOfWeek stores a day-of-week on its own. This stores a single day-of-week in isolation, such as ‘TUESDAY’.
Year stores a year on its own. This stores a single year in isolation, such as ‘2010’.
YearMonth stores a year and month without a day or time. This stores a year and month, such as ‘2010-12’ and could be used for a credit card expiry.
MonthDay stores a month and day without a year or time. This stores a month and day-of-month, such as ‘—12-03’ and could be used to store an annual event like a birthday without storing the year.
OffsetTime stores a time and offset from UTC without a date. This stores a date like ’11:30+01:00′. The ZoneOffset is of the form ‘+01:00’.
OffsetDateTime stores a date and time and offset from UTC. This stores a date-time like ‘2010-12-03T11:30+01:00’. This is sometimes found in XML messages and other forms of persistence, but contains less information than a full time-zone.
Package specification
Unless otherwise noted, passing a null argument to a constructor or method in any class or interface in this package will cause a NullPointerException to be thrown. The Javadoc «@param» definition is used to summarise the null-behavior. The «@throws NullPointerException » is not explicitly documented in each method.
All calculations should check for numeric overflow and throw either an ArithmeticException or a DateTimeException .
Design notes (non normative)
The API has been designed to reject null early and to be clear about this behavior. A key exception is any method that takes an object and returns a boolean, for the purpose of checking or validating, will generally return false for null.
- Instant — a timestamp
- LocalDate — a date without a time, or any reference to an offset or time-zone
- LocalTime — a time without a date, or any reference to an offset or time-zone
- LocalDateTime — combines date and time, but still without any offset or time-zone
- ZonedDateTime — a «full» date-time with time-zone and resolved offset from UTC/Greenwich
Instant is the closest equivalent class to java.util.Date . ZonedDateTime is the closest equivalent class to java.util.GregorianCalendar .
Where possible, applications should use LocalDate , LocalTime and LocalDateTime to better model the domain. For example, a birthday should be stored in a code LocalDate . Bear in mind that any use of a time-zone, such as ‘Europe/Paris’, adds considerable complexity to a calculation. Many applications can be written only using LocalDate , LocalTime and Instant , with the time-zone added at the user interface (UI) layer.
The offset-based date-time types OffsetTime and OffsetDateTime , are intended primarily for use with network protocols and database access. For example, most databases cannot automatically store a time-zone like ‘Europe/Paris’, but they can store an offset like ‘+02:00’.
Classes are also provided for the most important sub-parts of a date, including Month , DayOfWeek , Year , YearMonth and MonthDay . These can be used to model more complex date-time concepts. For example, YearMonth is useful for representing a credit card expiry.
Note that while there are a large number of classes representing different aspects of dates, there are relatively few dealing with different aspects of time. Following type-safety to its logical conclusion would have resulted in classes for hour-minute, hour-minute-second and hour-minute-second-nanosecond. While logically pure, this was not a practical option as it would have almost tripled the number of classes due to the combinations of date and time. Thus, LocalTime is used for all precisions of time, with zeroes used to imply lower precision.
Following full type-safety to its ultimate conclusion might also argue for a separate class for each field in date-time, such as a class for HourOfDay and another for DayOfMonth. This approach was tried, but was excessively complicated in the Java language, lacking usability. A similar problem occurs with periods. There is a case for a separate class for each period unit, such as a type for Years and a type for Minutes. However, this yields a lot of classes and a problem of type conversion. Thus, the set of date-time types provided is a compromise between purity and practicality.
- of — static factory method
- parse — static factory method focused on parsing
- get — gets the value of something
- is — checks if something is true
- with — the immutable equivalent of a setter
- plus — adds an amount to an object
- minus — subtracts an amount from an object
- to — converts this object to another type
- at — combines this object with another, such as date.atTime(time)
Multiple calendar systems is an awkward addition to the design challenges. The first principle is that most users want the standard ISO calendar system. As such, the main classes are ISO-only. The second principle is that most of those that want a non-ISO calendar system want it for user interaction, thus it is a UI localization issue. As such, date and time objects should be held as ISO objects in the data model and persistent storage, only being converted to and from a local calendar for display. The calendar system would be stored separately in the user preferences.
There are, however, some limited use cases where users believe they need to store and use dates in arbitrary calendar systems throughout the application. This is supported by ChronoLocalDate , however it is vital to read all the associated warnings in the Javadoc of that interface before using it. In summary, applications that require general interoperation between multiple calendar systems typically need to be written in a very different way to those only using the ISO calendar, thus most applications should just use ISO and avoid ChronoLocalDate .
The API is also designed for user extensibility, as there are many ways of calculating time. The field and unit API, accessed via TemporalAccessor and Temporal provide considerable flexibility to applications. In addition, the TemporalQuery and TemporalAdjuster interfaces provide day-to-day power, allowing code to read close to business requirements:
LocalDate customerBirthday = customer.loadBirthdayFromDatabase(); LocalDate today = LocalDate.now(); if (customerBirthday.equals(today))