T-sql: Round to nearest 15 minute interval

Handling common pauses and avoiding oversights in logic

Preventing overflow with future dates

Overflow can occur with a zero (0) date beyond 5500 years. Instead, let's select a future date:

-- "In the year 2000..." sings Conan O'Brien
SELECT DATEADD(MINUTE, ROUND(DATEDIFF(MINUTE, '20000101', @YourTime) / 15.0, 0) * 15, '20000101') AS RoundedTime
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This uses the floor of the millennium as the reference point - more Y2K compliant than a DJ at a millennium party!

Rounded to perfection: Nearest 30 seconds coming up

Want subminute granularity? Round to the nearest 30 seconds:

-- Smooth as a half-minute sand timer
SELECT DATEADD(SECOND, ROUND(DATEDIFF(SECOND, '20000101', @YourTime) / 30.0, 0) * 30, '20000101') AS RoundedTime
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With the fixed date well within SQL Server's abilities, you can have your seconds and round them too!

Applying time rounding on multiple columns

Working with starttime and stoptime? Apply the method to these datetime type columns:

SELECT 
  -- Time to start the music...
  DATEADD(MINUTE, ROUND(DATEDIFF(MINUTE, 0, starttime) / 15.0, 0) * 15, 0) AS RoundedStartTime,
  -- And stop the music!
  DATEADD(MINUTE, ROUND(DATEDIFF(MINUTE, 0, stoptime) / 15.0, 0) * 15, 0) AS RoundedStopTime
FROM YourTable
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Now your database is working like a well-oiled cuckoo clock, maintaining consistent rounding across columns.

Digging deeper: Rounding variations and patterns

Rounding down or up with the floor and ceiling functions

Firstly, for rounding down or up, we utilize FLOOR or CEILING respectively:

-- Down, down, deeper and down (Status Quo)
SELECT DATEADD(MINUTE, FLOOR(DATEDIFF(MINUTE, 0, @YourTime) / 15.0) * 15, 0) AS RoundedDownTime

-- Up, up and away (The 5th Dimension)
SELECT DATEADD(MINUTE, CEILING(DATEDIFF(MINUTE, 0, @YourTime) / 15.0) * 15, 0) AS RoundedUpTime
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Float like a butterfly, sting with precision

When you need to float like a butterfly, casting to float before rounding brings out the heavy artillery for maximum precision:

-- Data scientists be like ¯\_(ツ)_/¯
SELECT DATEADD(MINUTE, CAST((DATEDIFF(MINUTE, 0, CAST(@YourTime AS float)) / 15.0) AS INT) * 15, 0) AS RoundedTime
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Say bye-bye to seconds

When seconds aren't a factor and you want a simplified rounding technique, here it is:

-- 'Cause every minute counts but who's counting the seconds?
SELECT CAST(CAST(@YourTime AS smalldatetime) AS time) AS RoundedTimeApprox
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This rounds off seconds and offers a quick snap to the nearest minute.

Defusing time bombs: Merging into your existing systems

Remember to deftly update your existing T-SQL code when applying these methods:

-- Notice: The @YourTime field in your table will self-destruct in 5..4..3..2..1..
UPDATE YourTable
SET @YourTime = DATEADD(MINUTE, ROUND(DATEDIFF(MINUTE, 0, @YourTime) / 15.0, 0) * 15, 0)
WHERE some_condition
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A kind reminder: Thorough testing ensures your database won't go kaboom with the new changes!