The Biggest Problems with SQL Date Functions and How to Solve Them

Introduction

Date functions in SQL are powerful tools, but they are also one of the most common sources of errors in database queries and applications. Database administrators (DBAs) and developers frequently encounter issues with processing, storing, and converting date values. These problems often lead to inconsistent results, performance issues, or even data loss.

In this article, I will outline the most common challenges when working with SQL date functions, explain how they manifest, and provide possible solutions with their respective advantages and disadvantages.

1. Different Date Formats and Their Consequences

One of the biggest problems is the varying interpretation of date formats in SQL Server. While some languages and clients expect the YYYY-MM-DD format, others interpret DD-MM-YYYY. This can lead to unexpected results, especially when date values are stored as VARCHAR or NVARCHAR.

Example Problem

SELECT CAST('01-02-2023' AS DATE)

Depending on the SQL Server instance and regional settings, this can either return February 1, 2023, or January 2, 2023.

Solution

• Best Practice: Always use YYYYMMDD for DATE and DATETIME to avoid misinterpretations.

SELECT CAST('20230201' AS DATE)

• Alternatively: Explicitly specify a language or format option in the query.

SET LANGUAGE English;
SELECT CAST('01-02-2023' AS DATE);

📌 [Placeholder for Screenshot: Example of incorrect vs. correct results due to different date formats]

2. Issues with Time Zones and UTC Conversion

Many organizations operate distributed systems across different time zones. Without consistent time zone handling, problems with correct time calculations can quickly arise.

Symptoms

• Data shows unexpected shifts in DATETIME or DATETIMEOFFSET columns.
• Comparisons or calculations do not match expectations.

Solutions

• Always work with UTC:

SELECT GETUTCDATE();

• Convert time zones correctly using AT TIME ZONE:

-- Local time for Berlin (Germany)
SELECT 
    GETUTCDATE() AS UTC_Time,
    SYSDATETIMEOFFSET() AT TIME ZONE 'UTC' AT TIME ZONE 'Central European Standard Time' AS Berlin_LocalTime;

-- Local time for Seattle (USA)
SELECT 
    GETUTCDATE() AS UTC_Time,
    SYSDATETIMEOFFSET() AT TIME ZONE 'UTC' AT TIME ZONE 'Pacific Standard Time' AS Seattle_LocalTime;

Advantages:

✔ No dependency on server or session settings
✔ Automatic daylight saving time adjustments
✔ Correct conversions in distributed systems

Disadvantages:

❌ Increased storage requirements with DATETIMEOFFSET
❌ More complex queries for time zone calculations

📌 [Placeholder for Diagram: Example of UTC vs. local time shifts in a global application]

3. Calculation Errors with DATEDIFF and DATEADD

DATEDIFF is often used to calculate time spans, but it frequently returns unexpected results, particularly with DATETIME values.

Problem: Inaccurate Day Differences

Let’s say we want to calculate the difference between two timestamps:

SELECT DATEDIFF(DAY, '2023-02-28 23:59:59', '2023-03-01 00:00:00')

Expected: 1 day
Actual: 1 day (seems correct, but problematic in other cases)

A problematic case arises when a timestamp is close to a day boundary:

SELECT DATEDIFF(DAY, '2023-03-01 00:00:00', '2023-03-01 23:59:59')

Here, the result is 0, even though nearly a full day has passed. This is because DATEDIFF only counts the difference between whole days.

Problem: Incorrect Month Differences

Another issue occurs with month calculations:

SELECT DATEDIFF(MONTH, '2023-01-31', '2023-02-01')

Expected: 0, as only one day has passed.
Actual: 1, because the month value has changed.

Solution

• Exact difference calculation using seconds and conversion to days:

SELECT DATEDIFF(SECOND, '2023-01-01 23:59:59', '2023-01-02 00:00:01') / 86400.0 AS ExactDays;

• Using EOMONTH to correctly recognize month boundaries:

SELECT EOMONTH('2023-01-15');

Conclusion

SQL date and time functions are essential but often error-prone.

• Use consistent data types: DATETIME2 and DATETIMEOFFSET should be preferred over DATETIME for improved accuracy and better time zone management.
• Store timestamps in UTC: Storing timestamps in UTC and converting them to local time later is a best practice to avoid errors.
• Be cautious with calculations: DATEDIFF can return unexpected values, so more precise calculations using DATEDIFF(SECOND) or timestamp normalization may be better choices.
• Standardize date formats: Using YYYYMMDD as a string-based input for conversions reduces misinterpretations in different environments.

By being aware of these challenges and implementing best practices, you can manage SQL date values more efficiently and with fewer errors. Well-planned handling of date and time ensures consistent and maintainable database applications.

Björn Peters

Björn continues to work from Greece as a Senior Consultant – Microsoft Data Platform and Cloud for Kramer&Crew in Cologne. He also remains loyal to the community from his new home, he is involved in Data Saturdays or in various forums. Besides the topics around SQL Server, Powershell and Azure SQL, he is interested in science fiction, baking 😉 and cycling.