DateTime vs DateTimeOffset

Question

Currently  we have a standard way of dealing with  NET DateTime s in a TimeZone aware way  Whenever we produce a DateTime we do it in UTC  e g  using DateTime UtcNow   and whenever we display one  we convert back from UTC to the user s local time    That works fine  but I ve been reading about DateTimeOffset and how it captures the local and UTC time in the object itself  So the question is  what would be the advantages of using DateTimeOffset compared to what we have already been doing

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DateTimeOffset is a representation of instantaneous time  also known as absolute time    By that  I mean a moment in time that is universal for everyone  not accounting for leap seconds  or the relativistic effects of time dilation    Another way to represent instantaneous time is with a DateTime where  Kind is DateTimeKind Utc   This is distinct from calendar time  also known as civil time   which is a position on someone s calendar  and there are many different calendars all over the globe   We call these calendars time zones   Calendar time is represented by a DateTime where  Kind is DateTimeKind Unspecified  or DateTimeKind Local   And  Local is only meaningful in scenarios where you have an implied understanding of where the computer that is using the result is positioned    For example  a user s workstation   So then  why DateTimeOffset instead of a UTC DateTime   It s all about perspective   Let s use an analogy - we ll pretend to be photographers   Imagine you are standing on a calendar timeline  pointing a camera at a person on the instantaneous timeline laid out in front of you   You line up your camera according to the rules of your timezone - which change periodically due to daylight saving time  or due to other changes to the legal definition of your time zone    You don t have a steady hand  so your camera is shaky      The person standing in the photo would see the angle at which your camera came from  If others were taking pictures  they could be from different angles  This is what the Offset part of the DateTimeOffset represents   So if you label your camera  Eastern Time   sometimes you are pointing from -5  and sometimes you are pointing from -4   There are cameras all over the world  all labeled different things  and all pointing at the same instantaneous timeline from different angles   Some of them are right next to  or on top of  each other  so just knowing the offset isn t enough to determine which timezone the time is related to   And what about UTC   Well  it s the one camera out there that is guaranteed to have a steady hand   It s on a tripod  firmly anchored into the ground  It s not going anywhere   We call its angle of perspective the zero offset     So - what does this analogy tell us   It provides some intuitive guidelines-   If you are representing time relative to some place in particular  represent it in calendar time with a DateTime   Just be sure you don t ever confuse one calendar with another   Unspecified should be your assumption   Local is only useful coming from DateTime Now   For example  I might get DateTime Now and save it in a database - but when I retrieve it  I have to assume that it is Unspecified   I can t rely that my local calendar is the same calendar that it was originally taken from  If you must always be certain of the moment  make sure you are representing instantaneous time   Use DateTimeOffset to enforce it  or use UTC DateTime by convention  If you need to track a moment of instantaneous time  but you want to also know  What time did the user think it was on their local calendar   - then you must use a DateTimeOffset   This is very important for timekeeping systems  for example - both for technical and legal concerns  If you ever need to modify a previously recorded DateTimeOffset - you don t have enough information in the offset alone to ensure that the new offset is still relevant for the user   You must also store a timezone identifier   think - I need the name of that camera so I can take a new picture even if the position has changed    It should also be pointed out that Noda Time has a representation called ZonedDateTime for this  while the  Net base class library does not have anything similar   You would need to store both a DateTimeOffset and a TimeZoneInfo Id value  Occasionally  you will want to represent a calendar time that is local to  whomever is looking at it    For example  when defining what today means   Today is always midnight to midnight  but these represent a near-infinite number of overlapping ranges on the instantaneous timeline    In practice we have a finite number of timezones  but you can express offsets down to the tick   So in these situations  make sure you understand how to either limit the  who s asking   question down to a single time zone  or deal with translating them back to instantaneous time as appropriate    Here are a few other little bits about DateTimeOffset that back up this analogy  and some tips for keeping it straight    If you compare two DateTimeOffset values  they are first normalized to zero offset before comparing   In other words  2012-01-01T00 00 00 00 00 and 2012-01-01T02 00 00 02 00 refer to the same instantaneous moment  and are therefore equivalent  If you are doing any unit testing and need to be certain of the offset  test both the DateTimeOffset value  and the  Offset property separately  There is a one-way implicit conversion built in to the  Net framework that lets you pass a DateTime into any DateTimeOffset parameter or variable   When doing so  the  Kind matters   If you pass a UTC kind  it will carry in with a zero offset  but if you pass either  Local or  Unspecified  it will assume to be local   The framework is basically saying   Well  you asked me to convert calendar time to instantaneous time  but I have no idea where this came from  so I m just going to use the local calendar    This is a huge gotcha if you load up an unspecified DateTime on a computer with a different timezone    IMHO - that should throw an exception - but it doesn t     Shameless Plug   Many people have shared with me that they find this analogy extremely valuable  so I included it in my Pluralsight course  Date and Time Fundamentals   You ll find a step-by-step walkthrough of the camera analogy in the second module   Context Matters   in the clip titled  Calendar Time vs  Instantaneous Time

User · Answer

This piece of code from Microsoft explains everything      Find difference between Date Now and Date UtcNow   date1   DateTime Now    date2   DateTime UtcNow    difference   date1 - date2    Console WriteLine   0  -  1     2    date1  date2  difference         Find difference between Now and UtcNow using DateTimeOffset   dateOffset1   DateTimeOffset Now    dateOffset2   DateTimeOffset UtcNow    difference   dateOffset1 - dateOffset2    Console WriteLine   0  -  1     2                         dateOffset1  dateOffset2  difference        If run in the Pacific Standard time zone on 4 2 2007  the example      displays the following output to the console          4 2 2007 7 23 57 PM - 4 3 2007 2 23 57 AM   -07 00 00         4 2 2007 7 23 57 PM -07 00 - 4 3 2007 2 23 57 AM  00 00   00 00 00

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The only negative side of DateTimeOffset I see is that Microsoft  forgot   by design  to support it in their XmlSerializer class  But it has since been added to the XmlConvert utility class   XmlConvert ToDateTimeOffset  XmlConvert ToString   I say go ahead and use DateTimeOffset and TimeZoneInfo because of all the benefits  just beware when creating entities which will or may be serialized to or from XML  all business objects then

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DateTime is capable of storing only two distinct times  the local time and UTC   The Kind property indicates which   DateTimeOffset expands on this by being able to store local times from anywhere in the world   It also stores the offset between that local time and UTC   Note how DateTime cannot do this unless you d add an extra member to your class to store that UTC offset   Or only ever work with UTC   Which in itself is a fine idea btw

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There s a few places where DateTimeOffset makes sense  One is when you re dealing with recurring events and daylight savings time  Let s say I want to set an alarm to go off at 9am every day  If I use the  store as UTC  display as local time  rule  then the alarm will be going off at a different time when daylight savings time is in effect   There are probably others  but the above example is actually one that I ve run into in the past  this was before the addition of DateTimeOffset to the BCL - my solution at the time was to explicitly store the time in the local timezone  and save the timezone information along side it  basically what DateTimeOffset does internally

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Most of the answers are good   but i thought of adding some more links of MSDN for more information   A brief History of DateTime - By Anthony Moore by BCL team  Choosing between Datetime and DateTime Offset - by MSDN Do not forget SQL server 2008 onwards has a new Datatype as DateTimeOffset  The  NET Framework includes the DateTime  DateTimeOffset  and TimeZoneInfo types  all of which can be used to build applications that work with dates and times  Performing Arithmetic Operations with Dates and Times-MSDN

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The most important distinction is that DateTime does not store time zone information  while DateTimeOffset does   Although DateTime distinguishes between UTC and Local  there is absolutely no explicit time zone offset associated with it   If you do any kind of serialization or conversion  the server s time zone is going to be used   Even if you manually create a local time by adding minutes to offset a UTC time  you can still get bit in the serialization step  because  due to lack of any explicit offset in DateTime  it will use the server s time zone offset   For example  if you serialize a DateTime value with Kind Local using Json Net and an ISO date format  you ll get a string like 2015-08-05T07 00 00-04   Notice that last part  -04  had nothing to do with your DateTime or any offset you used to calculate it    it s just purely the server s time zone offset   Meanwhile  DateTimeOffset explicitly includes the offset   It may not include the name of the time zone  but at least it includes the offset  and if you serialize it  you re going to get the explicitly included offset in your value instead of whatever the server s local time happens to be

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A major difference is that DateTimeOffset can be used in conjunction with TimeZoneInfo to convert to local times in timezones other than the current one   This is useful on a server application  e g  ASP NET  that is accessed by users in different timezones

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From Microsoft      These uses for DateTimeOffset values are much more common than those for DateTime values  As a result  DateTimeOffset should be considered the default date and time type for application development    source   Choosing Between DateTime  DateTimeOffset  TimeSpan  and TimeZoneInfo   MSDN  We use DateTimeOffset for nearly everything as our application deals with particular points in time  e g  when a record was created updated   As a side note  we use DATETIMEOFFSET in SQL Server 2008 as well   I see DateTime as being useful when you want to deal with dates only  times only  or deal with either in a generic sense  For example  if you have an alarm that you want to go off every day at 7 am  you could store that in a DateTime utilizing a DateTimeKind of Unspecified because you want it to go off at 7am regardless of DST  But if you want to represent the history of alarm occurrences  you would use DateTimeOffset   Use caution when using a mix of DateTimeOffset and DateTime especially when assigning and comparing between the types  Also  only compare DateTime instances that are the same DateTimeKind because DateTime ignores timezone offset when comparing

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TLDR if you don t want to read all these great answers  -  Explicit  Using DateTimeOffset because the timezone is forced to UTC 0  Implicit  Using DateTime where you hope everyone sticks to the unwritten rule of the timezone always being UTC 0    Side note for devs  explicit is always better than implicit    Side side note for Java devs  C  DateTimeOffset    Java OffsetDateTime  read this  https   www baeldung com java-zoneddatetime-offsetdatetime

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