How to Add Days to a Date in T-SQL: Your Essential Guide for Time Manipulation

By Cristian G. Guasch • Updated: 09/23/23 • 9 min read

If you’re like me and find yourself working with SQL Server, there will come a time when you must manipulate dates in your database. This could entail adding or subtracting days from a specific date – a task that’s not immediately intuitive in T-SQL. The good news is, T-SQL provides robust functionality for handling these kinds of operations.

To add days to a date in T-SQL, we use the DATEADD function – a powerful tool built right into the language itself. It’s this function that’ll become your best friend when dealing with any sort of date manipulation. With it, you can add or subtract days, months, years – heck even minutes if that’s what you need.

In this article, I’ll guide you on how to use the DATEADD function to adeptly add days to dates within your SQL Server database. By the end of it all, you’ll feel confident manipulating dates with ease using T-SQL. So let’s jump right into it!

Understanding T-SQL and Date Manipulation

Diving headfirst into the world of T-SQL, it’s crucial to get a firm grasp on date manipulation. You might be asking yourself, “What is T-SQL?” Well, let me shed some light on that first. Transact SQL (T-SQL) is Microsoft’s proprietary extension of the SQL language. It’s packed with features that make it easier to manage databases in Microsoft SQL Server.

Now, when you’re working with data stored in an SQL database, chances are you’ll encounter dates. You see, manipulating dates in T-SQL can seem complicated at first glance but don’t worry – I’ve got your back! In fact, adding days to a date can become as easy as pie with just a few lines of code.

DECLARE @myDate DATE = '2022-01-01';
SET @myDate = DATEADD(DAY, 10, @myDate);
SELECT @myDate AS 'New Date';

In this bit of code snippet above what we’re doing is simple. We’re declaring a variable @myDate, assigning it a value (January 1st 2022), then using the DATEADD function to add ten days to that date.

Sounds simple enough right? However there are common pitfalls we need to avoid:

  • Always specify the unit argument (‘DAY’ in our example) accurately. It could be YEAR or MONTH or others.
  • Be sure that the start_date argument (@myDate here) isn’t NULL or an invalid date.
  • The number argument (10 in this case) mustn’t exceed certain limits based on your unit argument.

Don’t fret if this seems overwhelming now! The beauty of learning something new like T-SQL and its functions lies not just in understanding how they work but also becoming aware of their nuances and potential errors one might encounter. It’s like learning to dance; you’ve got to stumble a few times before you can glide smoothly across the floor!

Why Add Days to a Date in T-SQL?

Sometimes, I find myself working with databases where time plays a crucial role. In such cases, manipulating dates becomes an essential part of the work. One common operation is adding days to a date in T-SQL. But why would we want to do that? Let’s delve into it.

First off, by adding days to dates, you can predict future events based on historical data. For instance, if you’re running an e-commerce site and you notice that most customers make another purchase 30 days after their first one, it’d be beneficial for your marketing strategy to anticipate this pattern. With T-SQL’s DATEADD function, it’s easy as pie! Simply add 30 days to each customer’s initial purchase date and voila!

SELECT DATEADD(day, 30, OrderDate) AS 'Expected Next Purchase'
FROM Orders;

Secondly, adding days comes handy when calculating deadlines or expiration dates. If your business offers services with fixed durations like memberships or warranties – these are generally expressed in number of days. So again – just add those specific number of days onto the starting date using DATEADD. Here’s how:

SELECT DATEADD(day, MembershipDurationDays,
StartDate) AS 'Membership Expiration Date' FROM Memberships;

However bear in mind some typical pitfalls while doing so! Since T-SQL considers February with 28 days (non-leap year), adding one month will yield different results if done at end of January versus end of February. Instead always use day addition for accurate results across all scenarios.

Lastly but importantly: holidays and weekends! When planning tasks or projects based on business days only (excluding weekends and public holidays), simple day additions won’t suffice as they don’t account for non-working periods which may skew your calculations.

In conclusion (don’t worry – this isn’t the conclusion of our discussion), there are several reasons why you might want to add days to a date in T-SQL. Whether it’s for predicting customer behavior, calculating expiry dates, or scheduling tasks – adding days is a fundamental operation that you’ll find yourself using often. Just remember the nuances we talked about and you’re good to go!

Step-by-Step Guide on Adding Days to a Date in T-SQL

I’m here to guide you through the process of adding days to a date in T-SQL. Trust me, it’s simpler than you might think!

To kick things off, let’s take a quick peek at the basic syntax for adding days. Yes, there is an actual SQL command for this. It’s the DATEADD function and its simplicity will surely put your worries to rest.

Here’s how it works:

DATEADD (datepart , number , date )

In this syntax:

  • datepart is the part of date where number is added. This could be year, quarter, month, day etc.
  • number is the value that we wish to add.
  • date is the start date.

Let’s say we want to add 10 days to January 1, 2020. Here’s how you would do that:

SELECT DATEADD(day, 10, '2020-01-01') as NewDate;

The output would be ‘2020-01-11’. Easy peasy right?

There are common pitfalls you’ll want to avoid though. You might run into errors if you forget quotations around your date or misspell ‘day’ as ‘days’. Always remember – SQL can be quite picky about syntax!

Beyond just adding days with DATEADD function, T-SQL also gives us flexibility by allowing addition of other time units like months or years. For instance:

SELECT DATEADD(months, 3,'2022-08-15') as NewDate;

This would give us ‘2022-11-15’, three months ahead of our original date.

And there you have it! By following these steps and keeping an eye out for those common mistakes I’ve mentioned above (remember, SQL is picky!), you’ll be able to add days, months or even years to any date in T-SQL like a pro.

Common Mistakes When Adjusting Dates in T-SQL

Let’s dive into some of the common mistakes developers often make when adjusting dates using T-SQL. It’s not a surprise to stumble upon these errors, especially if you’re new to the language. But don’t worry, I’ll guide you through each one with code examples for a better understanding.

One typical pitfall is neglecting to account for leap years. You might be tempted to just add 365 days when trying to increment a year. However, this will lead you astray during leap years because they contain an extra day – February 29th.

For instance:

SELECT DATEADD(day, 365, '2020-02-29') AS NewDate

This command will return ‘2021-02-28’, not ‘2021-02-29’ as expected because 2021 isn’t a leap year.

A second frequent mistake is ignoring time components while working with dates. If you’re only interested in the date part and overlook the time component, it could cause unexpected results. Let’s say we have two datetime values:

DECLARE @dt1 datetime = '2019-01-01T14:23:00';
DECLARE @dt2 datetime = '2019-01-01T15:45:00';

If we try comparing these two using @dt1 + 1 = @dt2, it’ll fail since @dt2 also has a time aspect that makes it greater than @dt1.

Another stumbling block involves improper type conversions and data truncations which can lead to incorrect calculations or even runtime errors.

Here’s an example:


You’d expect this command would add three days to January 1st, but instead it throws an error. That’s because T-SQL attempts to convert the string ‘20190101’ directly to a datetime, which fails.

To avoid these common mistakes:

  • Always use DATEADD(YEAR, 1, @Date) when incrementing years.
  • Be aware of time components in your date variables.
  • Pay attention to type conversion and data truncation rules.

Remember that practice makes perfect. So keep honing your T-SQL skills and soon you’ll be handling dates like a pro!

Conclusion: Enhancing Your Database Skills with Date Operations

T-SQL is a powerful tool in the hands of any database professional. By learning how to add days to a date, you’ve taken another step towards mastering this versatile language.

Let’s take one last look at the steps involved:

  1. Start by selecting your desired date.
  2. Use the DATEADD function and specify ‘day’ as the datepart argument.
  3. Specify how many days you want to add.
  4. Finally, include your original date.

Here’s an example for clarity:

SELECT DATEADD(day, 5, '2022-01-01') AS NewDate;

In this example, we’re adding five days to January 1st, 2022.

It’s essential not to mix up the arguments in the DATEADD function – common mistake I’ve seen quite often! It’s always ‘datepart’, then number of units (in our case – days), and finally – start date.

Moreover, be careful when dealing with leap years or DST changes as these can sometimes lead to unexpected results!

Remember that practice makes perfect – apply these skills frequently in your projects and you’ll find yourself becoming more comfortable with T-SQL operations each day.

There are countless ways to use dates in SQL Server beyond simply adding or subtracting them – think about sorting data by month or determining someone’s age based on their birthday, for instance.

By mastering T-SQL’s built-in functions like DATEADD(), you’re laying a solid foundation for tackling even more complex problems down the road. Whether it’s generating reports or cleaning up data sets, there’s no telling what kind of challenges you’ll be able to overcome thanks to your newfound expertise!

Keep exploring, keep practicing and before long you’ll realize just how far you’ve come since dipping your toes into T-SQL waters!

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