When a PHP application needs to calculates the interval between two dates, choosing the right API and checking its results helps keep code understandable and dependable. This guide explains DateTimeImmutable::diff with focused examples and important details.
What Is DateTimeImmutable::diff in PHP?
DateTimeImmutable::diff calculates the interval between two dates. Calendar-day differences and total elapsed seconds differ around daylight-saving transitions.
Syntax:
DateTimeImmutable::diff(targetObject, absolute) : DateIntervalParameters and return value: Parameters: <required> DateTimeInterface $targetObject; <optional> bool $absolute = false. Check the return value or modified object rather than assuming the operation always succeeds.
Important: Use explicit IANA time zones in production and account for daylight-saving changes when calculating local schedules.
Example 1: Basic Usage
Start with a compact input so the main operation is visible.
<?php
$appointment = new DateTimeImmutable("2026-03-14 09:30:15.123456", new DateTimeZone("Europe/London"));
$deadline = new DateTimeImmutable("2026-03-20 12:00:00", new DateTimeZone("Europe/London"));
$interval = $appointment->diff($deadline);
echo $interval->format("%a days, %h hours");
Output:
6 days, 2 hoursCalculates the interval between two dates. Calendar-day differences and total elapsed seconds differ around daylight-saving transitions.
Example 2: A Different Input
The next example changes the input or option to illustrate another behavior.
<?php
$appointment = new DateTimeImmutable("2026-03-14 09:30:15.123456", new DateTimeZone("Europe/London"));
$earlier = new DateTimeImmutable("2026-03-01", new DateTimeZone("Europe/London"));
$interval = $appointment->diff($earlier);
echo $interval->invert;
Output:
1Compare this result with the first example. Calendar-day differences and total elapsed seconds differ around daylight-saving transitions.
Practical Example: Applying the Function
This example combines the API with a realistic processing step and shows where to check its result.
<?php
$start = new DateTimeImmutable("2026-10-24 12:00", new DateTimeZone("Europe/London"));
$end = new DateTimeImmutable("2026-10-26 12:00", new DateTimeZone("Europe/London"));
$gap = $start->diff($end);
echo $gap->format("%a calendar days; %h extra hours");
Output:
2 calendar days; 0 extra hoursAdapt the inputs to your application and validate the output before storing or displaying it. Calendar-day differences and total elapsed seconds differ around daylight-saving transitions.
Important Behavior and Edge Cases
- Main purpose: Calculates the interval between two dates.
- Errors and boundaries: Calendar-day differences and total elapsed seconds differ around daylight-saving transitions.
- Environment: Use explicit IANA time zones in production and account for daylight-saving changes when calculating local schedules.
Comparison With Related Functions
| Function or Approach | When to Use It |
|---|---|
DateTimeImmutable::diff | Calculates the interval between two dates. |
| DateInterval | Consider this alternative when you need different behavior or a different abstraction. |
Common Beginner Mistakes
| Mistake | Correct Approach |
|---|---|
| Assuming the operation succeeds for every input | Calendar-day differences and total elapsed seconds differ around daylight-saving transitions. |
| Ignoring runtime requirements or variable output | Use explicit IANA time zones in production and account for daylight-saving changes when calculating local schedules. |
| Using the wrong API for the goal | Compare DateTimeImmutable::diff with DateInterval before selecting an approach. |
Key Takeaways
- Calculates the interval between two dates.
- Calendar-day differences and total elapsed seconds differ around daylight-saving transitions.
- Use explicit IANA time zones in production and account for daylight-saving changes when calculating local schedules.
Use DateTimeImmutable::diff when you need to calculates the interval between two dates. The three examples move from a simple call to a practical use. Validate all external input and handle failures explicitly.