Hey Dev, are you struggling with datetime conversions in SQL Server? Don’t worry, you’re not alone! Converting datetime values can be a tricky task, especially when dealing with different time zones, formats, and cultures. In this article, we’ll walk you through the basics of datetime conversions in SQL Server, and provide you with helpful tips, code snippets, and real-world examples to make your job easier. So, let’s get started!
Understanding DateTime Data Type and Formats
Before we dive into the conversion techniques, let’s first understand what a datetime data type is and how it’s stored in SQL Server. In simple terms, datetime is a data type used to represent dates and times in SQL Server, with a precision of 3.33 milliseconds (or 1/300th of a second). The datetime data type includes both date and time values, separated by a space, and can be expressed in different formats depending on the cultural settings of the server or the user. Here are some common datetime formats in SQL Server:
Format |
Example |
Description |
|---|---|---|
YYYY-MM-DD |
2022-05-15 |
ISO date format |
YYYY-MM-DD HH:MI:SS |
2022-05-15 15:30:45 |
ISO datetime format |
MM/DD/YYYY |
05/15/2022 |
US date format |
MM/DD/YYYY HH:MI:SS |
05/15/2022 03:30:45 PM |
US datetime format |
DD/MM/YYYY |
15/05/2022 |
European date format |
DD/MM/YYYY HH:MI:SS |
15/05/2022 15:30:45 |
European datetime format |
It’s important to note that the datetime formats may vary depending on the language, culture, and regional settings of the system. Also, the datetime values are stored internally as floating-point numbers, with the integer part representing the number of days since January 1, 1900, and the fractional part representing the decimal fraction of the day (i.e., the time). This internal representation allows for arithmetic operations and comparisons on datetime values.
Dealing with Time Zones and Daylight Saving Time
One of the challenges of datetime conversions is dealing with time zones and daylight saving time. SQL Server stores datetime values in the local time zone of the server, which may not be the same as the time zone of the user or the application. To avoid confusion and errors, it’s recommended to always store datetime values in UTC (Coordinated Universal Time) format, which is a standard time reference used worldwide. UTC is equivalent to GMT (Greenwich Mean Time) and does not depend on the local time zone or the daylight saving time settings.
To convert a datetime value to UTC format, you can use the CONVERT function with the style code 120, which represents the ISO datetime format. For example:
SELECT CONVERT(datetime, '2022-05-15 15:30:45', 120) AT TIME ZONE 'UTC' AS utc_datetimeThis query will convert the datetime value ‘2022-05-15 15:30:45’ to UTC format, and return the result as a datetime value with the AT TIME ZONE clause. You can also use the GETUTCDATE function to get the current UTC datetime value, regardless of the time zone settings. For example:
SELECT GETUTCDATE() AS utc_datetimeThis query will return the current UTC datetime value as a datetime value.
Another challenge of datetime conversions is dealing with daylight saving time, which affects the local time zone offset and may cause datetime values to shift by one hour or more. To handle daylight saving time, you can use the DATENAME and DATEPART functions to extract the relevant parts of the datetime value, and adjust them accordingly. For example:
SELECT DATEADD(hour, CASE WHEN DATENAME(DW, @datetime) = 'Sunday'AND DATEPART(wk, @datetime) IN (10, 11)AND DATENAME(month, @datetime) = 'March'AND DATEPART(hh, @datetime) < 2THEN 1WHEN DATENAME(DW, @datetime) = 'Sunday'AND DATEPART(wk, @datetime) = 44AND DATENAME(month, @datetime) = 'October'AND DATEPART(hh, @datetime) >= 2THEN -1ELSE 0END, @datetime) AS local_datetimeThis query will adjust the datetime value @datetime to the local time zone, taking into account the daylight saving time rules for the US (where the clock moves forward by one hour on the second Sunday of March, and backward by one hour on the first Sunday of November). Note that this is just an example, and you may need to adjust the code for your specific time zone and daylight saving time rules.
Converting DateTime to String
Now that we’ve covered the basics of datetime formats and time zones, let’s move to the conversion techniques. The first conversion we’ll discuss is converting a datetime value to a string representation, which is often needed for display or export purposes. To convert a datetime value to a string, you can use the CONVERT or CAST functions, with a style code that represents the desired datetime format. Here’s an example:
SELECT CONVERT(varchar(20), @datetime, 120) AS datetime_stringThis query will convert the datetime value @datetime to a string representation in the ISO datetime format, and return the result as a varchar(20) value. You can also use the CAST function to achieve the same result, as follows:
SELECT CAST(@datetime AS varchar(20)) AS datetime_stringThis query will cast the datetime value @datetime to a varchar(20) value, without specifying a style code. In this case, the default format is used, which may vary depending on the language and regional settings of the system.
Dealing with Custom DateTime Formats
What if you need to convert a datetime value to a custom format, such as ‘MM/DD/YYYY HH:MI:SS AM/PM’? In this case, you can use the CONVERT function with a style code that represents the custom format. Here’s an example:
SELECT CONVERT(varchar(20), @datetime, 101) + ' '+ RIGHT(CONVERT(varchar(20), @datetime, 100), 7) AS datetime_stringThis query will convert the datetime value @datetime to a string representation in the ‘MM/DD/YYYY HH:MI:SS AM/PM’ format, and return the result as a varchar(20) value. The style code 101 represents the US date format, and the RIGHT function is used to extract the last 7 characters (i.e., the time part) of the datetime value, in the ‘HH:MI:SS’ format. Note that this is just an example, and you may need to adjust the code for your specific format requirements.
Converting String to DateTime
The second conversion we’ll discuss is converting a string representation to a datetime value, which is often needed for data validation or import purposes. To convert a string to a datetime, you can use the CONVERT or CAST functions, with a style code that represents the source datetime format. Here’s an example:
SELECT CONVERT(datetime, '05/15/2022 03:30:45 PM', 101) AS datetime_valueThis query will convert the string ’05/15/2022 03:30:45 PM’ to a datetime value, using the style code 101 that represents the US date format. You can also use the CAST function to achieve the same result, as follows:
SELECT CAST('05/15/2022 03:30:45 PM' AS datetime) AS datetime_valueThis query will cast the string ’05/15/2022 03:30:45 PM’ to a datetime value, without specifying a style code. In this case, the default format is used, which may vary depending on the language and regional settings of the system.
Dealing with Ambiguous DateTime Formats
What if the source datetime format is ambiguous, such as ‘MM/DD/YYYY’ or ‘DD/MM/YYYY’? In this case, you may need to use the SET DATEFORMAT command to specify the source format explicitly, before converting the string to a datetime. Here’s an example:
SET DATEFORMAT mdy;SELECT CONVERT(datetime, '05/15/2022', 101) AS datetime_valueThis query will set the ‘mdy’ date format, which represents the US date format, and then convert the string ’05/15/2022′ to a datetime value, using the style code 101. Note that you need to specify the SET DATEFORMAT command each time you want to convert a string with an ambiguous format. Also, be aware that the SET DATEFORMAT command may affect other queries or sessions that run concurrently, so use it with caution.
Converting DateTime to Date or Time
Sometimes you may need to extract only the date or the time part from a datetime value, for aggregation or filtering purposes. To convert a datetime value to a date or time value, you can use the CONVERT function with a style code that represents the desired format. Here are some examples:
SELECT CONVERT(date, @datetime) AS date_valueSELECT CONVERT(time, @datetime) AS time_valueThese queries will convert the datetime value @datetime to a date or time value, respectively, and return the result as a date or time data type. Note that the CONVERT function removes the time or date part from the datetime value, so the resulting value has a precision of 0 milliseconds.
Dealing with Date or Time to DateTime
If you want to combine a date or time value with a datetime value, you can use the DATEADD function or the plus (+) operator. Here are some examples:
SELECT DATEADD(day, DATEDIFF(day, 0, @date_value), @time_value) AS datetime_valueSELECT CAST(CAST(@date_value AS varchar(10)) + ' ' + CAST(@time_value AS varchar(8)) AS datetime) AS datetime_valueThese queries will combine the date value @date_value and the time value @time_value into a datetime value, using the DATEADD function and the CAST function, respectively. Note that in the second query, we first convert the date and the time values to varchar values, and then concatenate them with a space character, before casting the result to a datetime value.
Converting DateTime to Unix Timestamp
Unix timestamp is a way of representing dates and times as a single number, which is the number of seconds elapsed since January 1, 1970, at 00:00:00 UTC. Unix timestamp is widely used in programming and data exchange, and can be easily converted to datetime values using various tools and libraries. To convert a datetime value to a Unix timestamp, you can use the DATEDIFF function, which calculates the number of seconds between two datetime values. Here’s an example:
SELECT DATEDIFF(second, '19700101', @datetime) AS unix_timestampThis query will calculate the number of seconds between the datetime value @datetime and the Unix epoch (i.e., January 1, 1970, at 00:00:00 UTC), and return the result as an integer value. Note that some programming languages or tools may require the Unix timestamp to be in milliseconds or microseconds, so you may need to adjust the query accordingly.
Dealing with Unix Timestamp to DateTime
If you want to convert a Unix timestamp to a datetime value in SQL Server, you can use the DATEADD function, which adds a number of seconds, minutes, hours, or days to a datetime value. Here’s an example:
SELECT DATEADD(second, @unix_timestamp, '19700101') AS datetime_valueThis query will add the number of seconds @unix_timestamp to the Unix epoch (‘19700101’), and return the result as a datetime value. Note that the DATEADD function can add or subtract any number of seconds, so you can use negative values to subtract seconds from a datetime value.
Handling Invalid DateTime Values
Finally, it’s important to handle invalid datetime values properly, to avoid errors or unexpected results. Invalid datetime values can occur when converting strings with incorrect formats, or when performing arithmetic operations on datetime values that exceed the range of valid dates or times. To handle invalid datetime values, you can use the TRY_CONVERT function, which attempts to convert a value to a specified data type, and returns NULL if the conversion fails. Here’s an example:
SELECT TRY_CONVERT(datetime, '2022-05-32') AS datetime_valueThis query will attempt to convert the string ‘2022-05-32′ to a datetime value, using the TRY_CONVERT function. Since the date ’32’ is invalid, the function will return NULL instead of raising an error. You can also use the ISDATE function to check if a string can be converted to a datetime value, before applying the conversion. Here’s an example:
SELECT CASE WHEN ISDATE('2022-05-32') = 1THEN CONVERT(datetime, '2022-05-32')ELSE NULLEND AS datetime_valueThis query will check if the string ‘2022-05-32’ is a valid date (which it’s not), and if so, convert it to a datetime value. Otherwise, it will return NULL. Note that the ISDATE function may not detect all invalid dates or times, so use it with caution.
FAQ
What is the maximum and minimum datetime value in SQL Server?
The maximum datetime value in SQL Server is December 31, 9999, at 23:59:59.997, and the minimum datetime value is January 1, 1753, at 00:00:00.000. These values are represented internally as 2958465.99999999 and 0, respectively.
How can I convert a datetime value to a specific time zone?
To convert a datetime value to a specific time zone, you can use the AT TIME ZONE clause with a valid time zone name or abbreviation. For example:
SELECT @datetime AT TIME ZONE 'Central European Standard Time' AS central_datetimeThis query will convert the datetime value @datetime to the Central European Standard Time (CET) time zone, and return the result as a datetime value with the AT TIME ZONE clause. Note that the time zone name or abbreviation must be valid and supported by SQL Server.
How can I convert a datetime value to a specific culture or language?
To convert a datetime value to a specific culture or language, you can use the SET LANGUAGE and SET DATEFORMAT commands to set the desired language and format settings, before applying the conversion. For example:
SET LANGUAGE 'French';SET DATEFORMAT dmy;SELECT CONVERT(varchar(20), @datetime, 109) AS datetime_stringThis query will convert the datetime value @datetime to a string representation in the French culture, using the style code 109 that represents the European datetime format. Note that you need to specify the language and date format explicitly, and that the result may vary depending on the language and regional settings of the system.
Conclusion
In conclusion, datetime conversions in SQL Server can be a challenging but essential task for any developer or database administrator. By understanding the datetime data type, the various datetime formats