What s the difference between Instant and LocalDateTime

Question

I know that    Instant is rather a  technical  timestamp representation  nanoseconds  for computing  LocalDateTime is rather date clock representation including time-zones for humans    Still in the end IMO both can be taken as type for most application use-cases  As example  Currently I am running a batch job where I need to calculate a next run based on dates and I am struggling to find a pros cons between these two types  apart from the nanosecond precision advantage of Instant and the time-zone part of LocalDateTime    Can you name some application examples  where only Instant or LocalDateTime should be used   Edit  Beware misread documentations for LocalDateTime regarding precision and time-zone

User · Answer

Instant corresponds to time on the prime meridian  Greenwich    Whereas LocalDateTime relative to OS time zone settings  and     cannot represent an instant without additional information such as an offset or time-zone

User · Answer

One main difference is the Local part of LocalDateTime  If you live in Germany and create a LocalDateTime instance and someone else lives in USA and creates another instance at the very same moment  provided the clocks are properly set  - the value of those objects would actually be different  This does not apply to Instant  which is calculated independently from time zone   LocalDateTime stores date and time without timezone  but it s initial value is timezone dependent  Instant s is not   Moreover  LocalDateTime provides methods for manipulating date components like days  hours  months  An Instant does not      apart from the nanosecond precision advantage of Instant and the   time-zone part of LocalDateTime   Both classes have the same precision  LocalDateTime does not store timezone  Read javadocs thoroughly  because you may make a big mistake with such invalid assumptions  Instant and LocalDateTime

User · Answer

You are wrong about LocalDateTime  it does not store any time-zone information and it has nanosecond precision  Quoting the Javadoc  emphasis mine       A date-time without a time-zone in the ISO-8601 calendar system  such as 2007-12-03T10 15 30       LocalDateTime is an immutable date-time object that represents a date-time  often viewed as year-month-day-hour-minute-second  Other date and time fields  such as day-of-year  day-of-week and week-of-year  can also be accessed  Time is represented to nanosecond precision  For example  the value  2nd October 2007 at 13 45 30 123456789  can be stored in a LocalDateTime     The difference between the two is that Instant represents an offset from the Epoch  01-01-1970  and  as such  represents a particular instant on the time-line  Two Instant objects created at the same moment in two different places of the Earth will have exactly the same value

User · Answer

tl dr Instant and LocalDateTime are two entirely different animals  One represents a moment  the other does not   Instant represents a moment  a specific point in the timeline  LocalDateTime represents a date and a time-of-day  But lacking a time zone or offset-from-UTC  this class cannot represent a moment  It represents potential moments along a range of about 26 to 27 hours  the range of all time zones around the globe  A LocalDateTime value is inherently ambiguous   Incorrect Presumption  LocalDateTime is rather date clock representation including time-zones for humans   Your statement is incorrect  A LocalDateTime has no time zone  Having no time zone is the entire point of that class  To quote that class    doc   This class does not store or represent a time-zone  Instead  it is a description of the date  as used for birthdays  combined with the local time as seen on a wall clock  It cannot represent an instant on the time-line without additional information such as an offset or time-zone   So Local    means    not zoned  no offset     Instant  An Instant is a moment on the timeline in UTC  a count of nanoseconds since the epoch of the first moment of 1970 UTC  basically  see class doc for nitty-gritty details   Since most of your business logic  data storage  and data exchange should be in UTC  this is a handy class to be used often  Instant instant   Instant now         Capture the current moment in UTC   OffsetDateTime  The class OffsetDateTime class represents a moment as a date and time with a context of some number of hours-minutes-seconds ahead of  or behind  UTC  The amount of offset  the number of hours-minutes-seconds  is represented by the ZoneOffset class  If the number of hours-minutes-seconds is zero  an OffsetDateTime represents a moment in UTC the same as an Instant  ZoneOffset  The ZoneOffset class represents an offset-from-UTC  a number of hours-minutes-seconds ahead of UTC or behind UTC  A ZoneOffset is merely a number of hours-minutes-seconds  nothing more  A zone is much more  having a name and a history of changes to offset  So using a zone is always preferable to using a mere offset  ZoneId  A time zone is represented by the ZoneId class  A new day dawns earlier in Paris than in Montr  al  for example  So we need to move the clock   s hands to better reflect noon  when the Sun is directly overhead  for a given region  The further away eastward westward from the UTC line in west Europe Africa the larger the offset  A time zone is a set of rules for handling adjustments and anomalies as practiced by a local community or region  The most common anomaly is the all-too-popular lunacy known as Daylight Saving Time  DST   A time zone has the history of past rules  present rules  and rules confirmed for the near future  These rules change more often than you might expect  Be sure to keep your date-time library s rules  usually a copy of the  tz  database  up to date  Keeping up-to-date is easier than ever now in Java 8 with Oracle releasing a Timezone Updater Tool  Specify a proper time zone name in the format of Continent Region  such as America Montreal  Africa Casablanca  or Pacific Auckland  Never use the 2-4 letter abbreviation such as EST or IST as they are not true time zones  not standardized  and not even unique      Time Zone   Offset   Rules of Adjustments  ZoneId z   ZoneId of     Africa Tunis          ZonedDateTime  Think of ZonedDateTime conceptually as an Instant with an assigned ZoneId   ZonedDateTime     Instant   ZoneId    To capture the current moment as seen in the wall-clock time used by the people of a particular region  a time zone   ZonedDateTime zdt   ZonedDateTime now  z         Pass a  ZoneId  object such as  ZoneId of   quot Europe Paris quot        Nearly all of your backend  database  business logic  data persistence  data exchange should all be in UTC  But for presentation to users you need to adjust into a time zone expected by the user  This is the purpose of the ZonedDateTime class and the formatter classes used to generate String representations of those date-time values  ZonedDateTime zdt   instant atZone  z     String output   zdt toString                        Standard ISO 8601 format   You can generate text in localized format using DateTimeFormatter  DateTimeFormatter f   DateTimeFormatter ofLocalizedDateTime  FormatStyle FULL   withLocale  Locale CANADA FRENCH      String outputFormatted   zdt format  f       mardi 30 avril 2019    23 h 22 min 55 s heure de l   Inde  LocalDate  LocalTime  LocalDateTime    The  quot local quot  date time classes  LocalDateTime  LocalDate  LocalTime  are a different kind of critter  The are not tied to any one locality or time zone  They are not tied to the timeline  They have no real meaning until you apply them to a locality to find a point on the timeline  The word    Local    in these class names may be counter-intuitive to the uninitiated  The word means any locality  or every locality  but not a particular locality  So for business apps  the  quot Local quot  types are not often used as they represent just the general idea of a possible date or time not a specific moment on the timeline  Business apps tend to care about the exact moment an invoice arrived  a product shipped for transport  an employee was hired  or the taxi left the garage  So business app developers use Instant and ZonedDateTime classes most commonly  So when would we use LocalDateTime  In three situations   We want to apply a certain date and time-of-day across multiple locations  We are booking appointments  We have an intended yet undetermined time zone   Notice that none of these three cases involve a single certain specific point on the timeline  none of these are a moment  One time-of-day  multiple moments Sometimes we want to represent a certain time-of-day on a certain date  but want to apply that into multiple localities across time zones  For example   quot Christmas starts at midnight on the 25th of December 2015 quot  is a LocalDateTime  Midnight strikes at different moments in Paris than in Montr  al  and different again in Seattle and in Auckland  LocalDate ld   LocalDate of  2018   Month DECEMBER   25     LocalTime lt   LocalTime MIN        00 00 00 LocalDateTime ldt   LocalDateTime of  ld   lt         Christmas morning anywhere    Another example   quot Acme Company has a policy that lunchtime starts at 12 30 PM at each of its factories worldwide quot  is a LocalTime  To have real meaning you need to apply it to the timeline to figure the moment of 12 30 at the Stuttgart factory or 12 30 at the Rabat factory or 12 30 at the Sydney factory  Booking appointments Another situation to use LocalDateTime is for booking future events  ex  Dentist appointments   These appointments may be far enough out in the future that you risk politicians redefining the time zone  Politicians often give little forewarning  or even no warning at all  If you mean  quot 3 PM next January 23rd quot  regardless of how the politicians may play with the clock  then you cannot record a moment     that would see 3 PM turn into 2 PM or 4 PM if that region adopted or dropped Daylight Saving Time  for example  For appointments  store a LocalDateTime and a ZoneId  kept separately  Later  when generating a schedule  on-the-fly determine a moment by calling LocalDateTime  atZone  ZoneId   to generate a ZonedDateTime object  ZonedDateTime zdt   ldt atZone  z         Given a date  a time-of-day  and a time zone  determine a moment  a point on the timeline   If needed  you can adjust to UTC  Extract an Instant from the ZonedDateTime  Instant instant   zdt toInstant         Adjust from some zone to UTC  Same moment  same point on the timeline  different wall-clock time   Unknown zone Some people might use LocalDateTime in a situation where the time zone or offset is unknown  I consider this case inappropriate and unwise  If a zone or offset is intended but undetermined  you have bad data  That would be like storing a price of a product without knowing the intended currency  dollars  pounds  euros  etc    Not a good idea  All date-time types For completeness  here is a table of all the possible date-time types  both modern and legacy in Java  as well as those defined by the SQL standard  This might help to place the Instant  amp  LocalDateTime classes in a larger context   Notice the odd choices made by the Java team in designing JDBC 4 2  They chose to support all the java time times    except for the two most commonly used classes  Instant  amp  ZonedDateTime  But not to worry  We can easily convert back and forth  Converting Instant     Storing OffsetDateTime odt   instant atOffset  ZoneOffset UTC     myPreparedStatement setObject        odt         Retrieving OffsetDateTime odt   myResultSet getObject        OffsetDateTime class     Instant instant   odt toInstant      Converting ZonedDateTime     Storing OffsetDateTime odt   zdt toOffsetDateTime     myPreparedStatement setObject        odt         Retrieving OffsetDateTime odt   myResultSet getObject        OffsetDateTime class     ZoneId z   ZoneId of   quot Asia Kolkata quot      ZonedDateTime zdt   odt atZone  z        About java time The java time framework is built into Java 8 and later  These classes supplant the troublesome old legacy date-time classes such as java util Date  Calendar   amp  SimpleDateFormat  To learn more  see the Oracle Tutorial  And search Stack Overflow for many examples and explanations  Specification is JSR 310  The Joda-Time project  now in maintenance mode  advises migration to the java time classes  You may exchange java time objects directly with your database  Use a JDBC driver compliant with JDBC 4 2 or later  No need for strings  no need for java sql   classes  Hibernate 5  amp  JPA 2 2 support java time  Where to obtain the java time classes   Java SE 8  Java SE 9  Java SE 10  Java SE 11  and later  - Part of the standard Java API with a bundled implementation   Java 9 brought some minor features and fixes    Java SE 6 and Java SE 7  Most of the java time functionality is back-ported to Java 6  amp  7 in ThreeTen-Backport    Android  Later versions of Android  26   bundle implementations of the java time classes  For earlier Android   lt 26   a process known as API desugaring brings a subset of the java time functionality not originally built into Android   If the desugaring does not offer what you need  the ThreeTenABP project adapts ThreeTen-Backport  mentioned above  to Android  See How to use ThreeTenABP           The ThreeTen-Extra project extends java time with additional classes  This project is a proving ground for possible future additions to java time  You may find some useful classes here such as Interval  YearWeek  YearQuarter  and more

[java] What's the difference between Instant and LocalDateTime?

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