When working with PHP, you may need to compute the calendar difference between two dates. This guide explains the operation, shows what its arguments mean, and uses three separate examples to make the result easier to understand.
date_diff: Returns an interval describing the relationship between two dates, with an invert flag when relevant.
What Is date_diff in PHP?
Compute the calendar difference between two dates. Returns an interval describing the relationship between two dates, with an invert flag when relevant.
Syntax:
date_diff(DateTimeInterface $baseObject, DateTimeInterface $targetObject, bool $absolute = false): DateInterval
Parameters and return value: The signature above identifies the arguments and return type where the API has a single call. For topic guides, follow the sequence of calls in the examples. Always distinguish a returned value from an object or variable modified by reference.
Important: Calendar differences are sensitive to timezones and daylight-saving transitions; %a is total days, %d is only the day component.
Example 1: Basic Usage
This example introduces the operation with a short and predictable input.
<?php
$first = new DateTimeImmutable("2026-10-01");
$second = new DateTimeImmutable("2026-10-16");
echo date_diff($first, $second)->format("%a days");
Output:
15 days
The code focuses on the main operation. Returns an interval describing the relationship between two dates, with an invert flag when relevant.
Example 2: Another Case
Now compare a different input, locale, date, or option to see another part of the behavior.
<?php
$first = new DateTimeImmutable("2026-10-16");
$second = new DateTimeImmutable("2026-10-01");
echo date_diff($first, $second)->format("%R%a days");
Output:
-15 days
What changes? Calendar differences are sensitive to timezones and daylight-saving transitions; %a is total days, %d is only the day component.
Practical Example: Applying It in an Application
Use the following separate example as a starting point for a small application task. Check any extension or environment requirements below before running it.
<?php
$start = new DateTimeImmutable("2026-01-01", new DateTimeZone("UTC"));
$milestones = ["2026-02-01", "2026-03-15"];
foreach ($milestones as $milestone) {
$difference = date_diff($start, new DateTimeImmutable($milestone, new DateTimeZone("UTC")));
echo $milestone . ": " . $difference->format("%a days") . PHP_EOL;
}
Output:
2026-02-01: 31 days
2026-03-15: 73 days
The practical example demonstrates how to apply this operation to multiple values, a reusable task, or a validated input. Returns an interval describing the relationship between two dates, with an invert flag when relevant.
Important Behavior and Edge Cases
- Core behavior: Returns an interval describing the relationship between two dates, with an invert flag when relevant.
- Inputs and results: Calendar differences are sensitive to timezones and daylight-saving transitions; %a is total days, %d is only the day component.
- Alternatives: DateTimeInterface::diff() is the object-oriented equivalent.
Comparison With Related Functions
| Function or Approach | When to Use It |
|---|---|
date_diff | Compute the calendar difference between two dates. |
| Related PHP approach | DateTimeInterface::diff() is the object-oriented equivalent. |
Common Beginner Mistakes
| Mistake | Correct Approach |
|---|---|
| Using an incorrect input or option | Calendar differences are sensitive to timezones and daylight-saving transitions; %a is total days, %d is only the day component. |
| Confusing this API with a related operation | DateTimeInterface::diff() is the object-oriented equivalent. |
| Assuming output is identical across environments | Check return values and required extensions or versions. Use explicit locale, timezone or encoding settings when they affect the result. |
Key Takeaways
- date_diff helps you compute the calendar difference between two dates.
- Returns an interval describing the relationship between two dates, with an invert flag when relevant.
- Calendar differences are sensitive to timezones and daylight-saving transitions; %a is total days, %d is only the day component.
Use date_diff when you need to compute the calendar difference between two dates. Start with the first example, compare the second, and adapt the practical example to your inputs. Confirm the return value and environment requirements before adding it to production code.