DATEDIFF() Returns Wrong Results in SQL Server? Read This.

Posted on by Ian

If you’re getting some really weird results when using the DATEDIFF() function in SQL Server, and you’re convinced the function contains a bug, don’t tear your hair out just yet. It’s probably not a bug.

There are scenarios where the results produced by this function can be pretty whacky. And if you don’t understand how the function actually works, the results will look completely wrong.

Hopefully this article can help clarify how the DATEDIFF() function is designed to work, and provide some example scenarios of where your results might not be as you’d expect.

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A Workaround for DATEDIFF() Ignoring SET DATEFIRST in SQL Server (T-SQL Example)

Posted on by Ian

An interesting thing about the DATEDIFF() function in SQL Server is that it ignores your SET DATEFIRST value.

However, this is not a bug. Microsoft’s documentation for DATEDIFF() clearly states the following:

Specifying SET DATEFIRST has no effect on DATEDIFF. DATEDIFF always uses Sunday as the first day of the week to ensure the function operates in a deterministic way.

In case you don’t know, SET DATEFIRST sets the first day of the week for your session. It’s a number from 1 through 7 (which corresponds to Monday through Sunday).

The initial value for SET DATEFIRST is implicitly set by the language setting (which you can set with the SET LANGUAGE statement). The actual value will depend on the language that is set. For example the default value for the us_english language is 7 (Sunday), whereas the default for the British language is 1 (Monday).

However, you can use a SET DATEFIRST statement to override this so that you can keep using the same language while using a different day for the first day of the week.

But as mentioned, the SET DATEFIRST value has no effect on the DATEDIFF() function. The DATEDIFF() function always assumes that Sunday is the first day of the week regardless of your SET DATEFIRST value.

This can cause some interesting problems when using DATEDIFF() if you don’t know how it works.

If you find yourself in this situation, hopefully the examples on this page can help.

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Convert ‘time’ to ‘datetime2’ in SQL Server (T-SQL Examples)

Posted on by Ian

This article contains examples of converting a time value to a datetime2 value in SQL Server.

When you convert a time value to datetime2, extra information is added to the value. This is because the datetime2 data type contains both date and time information. The time data type, on the other hand, only contains time information.

More specifically, the date is set to ‘1900-01-01’ (unless it happens to get rounded up to ‘1900-01-02’), the time component is copied, and according to the Microsoft documentation, the time zone offset is set to 00:00 (even though the datetime2 data type is not time zone aware and doesn’t preserve any time zone offset).

When the fractional seconds precision of the datetime2(n) value is greater than the time(n) value, the value is rounded up.

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Convert ‘time’ to ‘datetimeoffset’ in SQL Server (T-SQL Examples)

Posted on by Ian

This article contains examples of converting a time value to a datetimeoffset value in SQL Server using Transact-SQL.

When you convert a time value to datetimeoffset, the date is set to ‘1900-01-01’ and the time is copied. A time zone offset is added and set to +00:00.

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MySQL TIMEDIFF() vs TIMESTAMPDIFF(): What’s the Difference?

Posted on by Ian

This article looks at the difference between the MySQL TIMEDIFF() and TIMESTAMPDIFF() functions.

Both functions do a similar thing, but there are some significant differences between the two.

The following table summarizes the difference between these two functions:

TIMEDIFF() TIMESTAMPDIFF()
Requires 2 arguments. Requires 3 arguments.
Subtracts the 2nd argument from the 1st (date1 − date2). Subtracts the 2nd argument from the 3rd (date2 − date1).
Result is expressed as a time value (and it has the limitations of the time data type). Result is an integer, expressed by a number of units as provided by the first argument.
Accepts time or datetime expressions. Accepts date or datetime expressions.
Both arguments must be the same type (either time or datetime). Both arguments can be of a different type (date or datetime).

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MySQL DATEDIFF() vs TIMESTAMPDIFF(): What’s the Difference?

Posted on by Ian

This article looks at the difference between two MySQL functions; DATEDIFF() and TIMESTAMPDIFF().

Both functions return the difference between two dates and/or times, but the result is different between the two functions.

The following table summarizes the difference between these two functions:

DATEDIFF() TIMESTAMPDIFF()
Requires 2 arguments. Requires 3 arguments.
Subtracts the 2nd argument from the 1st (expr1 − expr2). Subtracts the 2nd argument from the 3rd (expr2 − expr1).
Result is expressed as a value in days. Result is expressed as the unit provided by the first argument.
Can compare only the date value of its arguments. Can compare the date and time value of its arguments.

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MySQL DATEDIFF() vs TIMEDIFF(): What’s the Difference?

Posted on by Ian

Two date functions included in MySQL are DATEDIFF() and TIMEDIFF().

Both functions do a similar thing, but with some meaningful differences.

The following table summarizes the difference between these two functions:

DATEDIFF() TIMEDIFF()
Result is expressed as a value in days. Result is expressed as a time value.
Compares only the date value of its arguments. Compares the time value of its arguments.
Accepts date or date-and-time expressions. Accepts time or date-and-time expressions.
Both arguments can be of a different type (date or date-and-time). Both arguments must be the same type (either time or date-and-time).

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DATEDIFF() vs DATEDIFF_BIG() in SQL Server: What’s the Difference?

Posted on by Ian

If you’ve ever needed to find the difference between two dates in SQL Server, you might have used the DATEDIFF() function. This function returns the amount of time between two dates using a datepart specified by you. For example, you could use it to return the number of days between date 1 and date 2. You can also get it to return the number of minutes, seconds, months, years, etc.

The DATEDIFF_BIG() function works exactly the same way, but with one subtle difference: Its return data type.

So the difference between these two functions is the data type of their return value.

  • DATEDIFF() returns a signed integer (int)
  • DATEDIFF_BIG() returns a signed big integer (bigint)

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What is the Year 2038 Problem?

Posted on by Ian

The Year 2038 problem (also referred to as the Y2K38 bug) refers to a problem that some computer systems might encounter when dealing with times past 2038-01-19 03:14:07.

Many computer systems, such as Unix and Unix-based systems, don’t calculate time using the Gregorian calendar. They calculate time as the number of seconds since 1 January 1970. Therefore, in these systems, time is represented as a big number (i.e. the number of seconds passed since 1970-01-01 00:00:00). This is typically referred to as Epoch time, Unix time, Unix Epoch time, or POSIX time. As I write this, Unix time is 1560913841. And as I write this next line, Unix time has incremented to 1560913879.

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