DateTime vs DateTimeOffset
What is the difference between a DateTime and a DateTimeOffset and when should one be used?
Currently, we have a standard way of dealing with .NET DateTimes 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.
回答1
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.
回答2
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)
Compare types related to date and time | Microsoft Learn
The DateTimeOffset structure
The DateTimeOffset structure represents a date and time value, together with an offset that indicates how much that value differs from UTC. Thus, the value always unambiguously identifies a single point in time.
The DateTimeOffset type includes all of the functionality of the DateTime type along with time zone awareness. This makes it suitable for applications that:
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Uniquely and unambiguously identify a single point in time. The DateTimeOffset type can be used to unambiguously define the meaning of "now", to log transaction times, to log the times of system or application events, and to record file creation and modification times.
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Perform general date and time arithmetic.
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Preserve multiple related times, as long as those times are stored as two separate values or as two members of a structure.
Note
These uses for DateTimeOffset values are much more common than those for DateTime values.
As a result, consider DateTimeOffset as the default date and time type for application development.
Yes, it's true that DateTimeOffset is often preferred over DateTime as the default date and time type for application development, especially in scenarios where you need to store or manipulate date and time values that are associated with a specific time zone or offset. This is because DateTimeOffset represents an unambiguous point in time with respect to a specific offset from Coordinated Universal Time (UTC), whereas DateTime does not include any information about time zone or offset, and is therefore more ambiguous and prone to errors when working with time zone conversions and daylight saving time transitions.
Furthermore, many modern APIs and frameworks, including the .NET framework, have been updated to use DateTimeOffset by default, which makes it easier to work with date and time values across different time zones and daylight saving time transitions. However, there may be some scenarios where DateTime is still a better choice, such as when you need to work with legacy systems or databases that do not support DateTimeOffset, or when you need to perform high-performance operations on large datasets that do not require time zone information.
What's the default datetime type of js? datetime or datetimeoffset
JavaScript does not have a built-in DateTime or DateTimeOffset type. Instead, it has a built-in Date object, which represents a point in time. The Date object stores the number of milliseconds elapsed since January 1, 1970, 00:00:00 UTC, which is also known as the Unix epoch.
The Date object does not have built-in support for time zones, so it is up to the developer to handle time zone conversions as needed. However, JavaScript does provide the Intl.DateTimeFormat object, which can be used to format dates in different time zones and locales. Additionally, there are many third-party libraries available for handling dates and time zones in JavaScript, such as Moment.js and Luxon.
How does moment.js handle datetimeoffset?
Moment.js is a popular JavaScript library for working with dates and times. By default, Moment.js handles dates and times in the local time zone of the user's browser.
However, Moment.js also provides a plugin called moment-timezone, which extends Moment.js to support time zones. With moment-timezone, you can create a moment object with a specific time zone offset, like so:
const moment = require('moment-timezone');
// create a moment object with a specific offset
const m = moment.utc('2022-01-05T12:34:56.789-05:00');utc function to indicate that the input time should be interpreted as UTC, and then we're using the moment-timezone plugin to apply the time zone offset.Once you have a moment object with a specific time zone offset, you can format it for display in various ways, such as:
// format the moment object as a string in a specific time zone
const s = m.tz('Asia/Shanghai').format('YYYY-MM-DD HH:mm:ss');
console.log(s); // prints "2022-01-06 01:34:56"In this example, we're formatting the moment object as a string in the Asia/Shanghai time zone using the tz function. We're then using the format function to specify the format of the output string. The resulting string represents the same moment in time, but with a different time zone offset.
Please give me some sample, which other languages use DateTimeOffset by default?
Some programming languages and frameworks that use DateTimeOffset or similar types as the default for handling date and time values include:
- Java: The java.time package introduced in Java 8 includes the ZonedDateTime class, which stores date and time values along with a time zone offset or zone ID.
- Ruby: The DateTime class in Ruby includes support for time zone offsets and is often used alongside the ActiveSupport::TimeWithZone class, which provides additional features for working with time zones.
- Python: The pytz library is commonly used in Python applications for working with time zones and daylight saving time transitions, and the datetime class in the standard library includes support for time zone offsets.
These are just a few examples, and there are many other languages and frameworks that support working with DateTimeOffset or similar types.
index efficiency difference between datetime2 and datetimeoffset in sql server?
In SQL Server, datetime2 and datetimeoffset are both highly efficient data types for indexing and querying date and time values. However, there are some differences in their efficiency based on how they are used.
The datetime2 data type has a fixed size of 6 to 8 bytes, depending on the precision specified, and is stored as an 8-byte floating-point number. This makes it efficient for indexing and querying on the basis of date and time values, especially when high precision is not needed.
The datetimeoffset data type, on the other hand, has a fixed size of 10 bytes, which includes both the date and time values as well as the offset value. This makes it slightly less efficient than datetime2 for indexing and querying on the basis of date and time values alone. However, datetimeoffset is preferred when working with data that needs to be represented in a specific time zone or when you need to handle daylight saving time transitions.
In general, the efficiency difference between datetime2 and datetimeoffset is negligible for most applications, and the choice of data type should be based on the specific requirements of the application.