Working with time data comes up in real systems all the time. Applications record when an order is placed, when an event happens, or how long a process takes. To turn that information into something useful, you often need to compare two timestamps and see the gap between them. Databases handle this with built-in functions and arithmetic that involve the INTERVAL type. The exact syntax changes depending on the database, but the idea is the same. The system breaks down the timestamps into their parts, runs the comparison, and gives you the difference in the unit you asked for.
How Databases Calculate Differences Between Timestamps
Every database that handles time data has to translate human-readable dates into something it can measure. A value like 2025-09-02 14:30:00 looks very basic to us, but the system needs to turn that into numbers it can compare with another timestamp. When a subtraction is performed, the database reduces both timestamps into a shared numeric unit and then computes the gap. After that, the difference is either returned directly or wrapped in a specialized type called INTERVAL.
Storage Mechanics of Timestamps
MySQL stores DATETIME and TIMESTAMP values in a binary layout. Each field (year, month, day, hour, minute, second, fractional second) is held in a compact structure. When you ask MySQL to compare two timestamps, it converts both into a count of microseconds before subtraction. That conversion makes sure that fractional seconds are accurate down to six decimal places.
Running this query confirms that MySQL holds microsecond-level detail. The subtraction of two such values relies on that stored precision.
PostgreSQL takes a different path by representing timestamps as the number of microseconds from a fixed point in time, January 1, 2000. This makes arithmetic straightforward because subtraction reduces to integer math on very large numbers.
The query returns a fraction of a second, which reflects the underlying integer math on microseconds.
SQL Server stores datetime2 as two separate numeric parts: days since 0001-01-01 and fractions of a day. Internally SQL Server stores the date as the number of days and the time as fractional ticks of a day, and it works out differences by comparing those components.
Direct subtraction with - isn’t supported for datetime2. Use DATEDIFF or DATEDIFF_BIG to compute differences.
In Oracle, subtracting DATE values returns a numeric day count, while subtracting TIMESTAMP values returns an INTERVAL DAY TO SECOND. That makes it flexible for multiplication into hours, minutes, or seconds.
Conversion Into Intervals
After timestamps are stored and reduced to numbers, the database needs to present the result in a way that’s practical for queries. Some systems return an interval type, others return a number you can manipulate further.
PostgreSQL subtraction directly yields an interval. That type is capable of expressing differences that span months, days, and smaller units. It allows you to ask for very specific parts of the difference.
The result is reported as 1 day 1:30:00, which represents the full duration across days and hours. That interval object can then be queried for its pieces.
MySQL prefers a function-driven method rather than returning an interval directly. Internally, subtraction still happens in microseconds, but you request the final unit with TIMESTAMPDIFF. The difference is that the conversion into a friendly number happens inside the function instead of being returned as an interval object.
SQL Server uses DATEDIFF to return integers in the requested unit. Its arithmetic works with offsets of days and fractional ticks of a day, then the function divides by the appropriate unit before producing the integer result.
Oracle subtraction produces a number of days by default. That makes it flexible because you can multiply the result by 24, 1440, or 86400 to get hours, minutes, or seconds.
The query returns 1.0625, which corresponds to 1 day and 1.5 hours. Multiplying that by 24 would yield 25.5 hours.
Practical Methods To Get Differences In Minutes Hours Or Days
Different database engines expose their own functions for turning raw timestamp differences into values that are easy to work with. The mechanics of subtraction are the same in the background, but each system offers its own way of packaging the result. Some provide a specialized function that handles the math and returns the number in a single call, while others return an interval object that you can query further.
MySQL With TIMESTAMPDIFF
MySQL relies on the TIMESTAMPDIFF function for timestamp arithmetic. The function takes three arguments: the unit you want, the starting value, and the ending value. Internally, both timestamps are converted into microseconds, the subtraction is performed, and the system divides the difference by the length of the unit you asked for.
MySQL is strict about the order of arguments. Reversing them flips the sign of the result. That’s important when queries are used for things like finding overdue events, since a negative number will indicate that the first argument is later than the second.
PostgreSQL With Subtraction And EXTRACT
PostgreSQL returns an interval type whenever two timestamps are subtracted. That interval can express differences in multiple parts at once, such as days and hours. To get a specific numeric result like minutes, you can pull the total number of seconds from the interval with EXTRACT(EPOCH FROM interval) and then divide.
PostgreSQL’s method is flexible. You can work directly with the interval when you want a readable difference, or convert it into pure numbers when building metrics or reports. The interval type can store months and years, but subtraction of two timestamps yields a day-to-second interval. To express differences in months or years, use age(ts_end, ts_start).
SQL Server With DATEDIFF
SQL Server provides the DATEDIFF function, which has a similar structure to MySQL’s TIMESTAMPDIFF. You specify a unit, a start point, and an end point. Behind the scenes, SQL Server converts timestamps into day counts and fractional ticks of a day before subtracting them.
The result is always an integer. That means fractional parts are not returned. If two timestamps are 89 minutes apart, DATEDIFF(MINUTE, start_time, end_time) gives back 89 rather than 89.5.
SQL Server measures differences by counting how many datepart boundaries get crossed. If two values fall within the same boundary the count stays at 0, but once the end value passes into the next boundary the count moves up by 1.
The answer is 1 because DATEDIFF counts the number of second boundaries crossed, not fractional seconds within the same boundary.
Oracle With Interval Arithmetic
In Oracle, subtracting DATE values directly returns a number measured in days. That number includes fractional parts, so multiplying gives you values in hours, minutes, or seconds.
Because DATE subtraction returns decimals for day differences, you get precision across smaller units without needing extra functions. Multiplying by 24 produces hours with fractional values included. That fractional part can be valuable when working with durations that don’t line up perfectly with whole numbers of hours. Oracle also supports explicit INTERVAL types when working with TIMESTAMP values, but for daily work with DATE arithmetic, the numeric result is often enough.
Conclusion
Every database engine follows the same pattern when it needs to subtract timestamps, even if the surface details look different. The system first reduces both values to exact numeric units such as microseconds, day counts, or fractional ticks. That raw difference is then scaled into something practical like minutes, hours, or days. Some databases return an interval object that can be queried further, while others hand back integers or decimals that you can adjust as needed. What matters is that the mechanics stay consistent, the timestamps are broken into precise values, compared, and expressed in a way that lets queries capture real spans of time accurately.
