CarbonImmutable in Laravel: prevent date mutation bugs

Use CarbonImmutable when a date is reused across calculations. Methods such as addDay() return a new value, leaving the original unchanged. Mutable Carbon changes the original object instead. Laravel can use immutable dates through its Date facade and Eloquent casts.

Why Mutable Dates Cause Problems

With standard Carbon, every modification mutates the original:

$start = Carbon::parse('2024-06-01');
$end   = $start->addDays(7); // $start is now also 2024-06-08

echo $start->format('Y-m-d'); // 2024-06-08 — not what you expected
echo $end->format('Y-m-d');   // 2024-06-08

This is a classic source of bugs, especially when a date variable is shared across multiple calculations.

CarbonImmutable Returns New Instances

CarbonImmutable works identically to Carbon, but every method that would normally mutate the instance returns a brand new one instead:

use Carbon\CarbonImmutable;

$start = CarbonImmutable::parse('2024-06-01');
$end   = $start->addDays(7); // $start is untouched

echo $start->format('Y-m-d'); // 2024-06-01 — safe
echo $end->format('Y-m-d');   // 2024-06-08

Configuring Laravel to Use CarbonImmutable

Laravel provides a helper to switch the default date class application-wide. Add this to your AppServiceProvider:

use Illuminate\Support\Facades\Date;
use Carbon\CarbonImmutable;
use Illuminate\Support\ServiceProvider;

class AppServiceProvider extends ServiceProvider
{
    public function boot(): void
    {
        Date::use(CarbonImmutable::class);
    }
}

This configures Laravel's Date factory, used by helpers such as now(). PHP's date() function still returns a formatted string. Direct Carbon::now() calls still use Carbon. Configure Eloquent casts explicitly when a model attribute must be immutable.

Immutable Eloquent date casts

For a model-specific choice, set its date casts explicitly:

protected function casts(): array
{
    return [
        'created_at' => 'immutable_datetime',
        'updated_at' => 'immutable_datetime',
    ];
}

Assign the result of a date calculation when it should be saved: $model->created_at = $model->created_at->addDay(); Calling addDay() alone leaves an immutable value unchanged.

Laravel's Date Helpers

Laravel ships with several helper functions that respect the configured date class:

// Returns the configured date class (Carbon or CarbonImmutable)
$now = now();

// Parse a date string using the configured class
$date = Date::parse('2024-12-25');

// Create from a format
$date = Date::createFromFormat('d/m/Y', '25/12/2024');

Prefer these helpers over calling Carbon::now() or CarbonImmutable::now() directly. They ensure your application stays consistent if you ever change the default date class.

Practical Example

A common pattern is calculating date ranges for reports:

// ❌ With mutable Carbon — the bug
$rangeStart = Carbon::now()->startOfMonth();
$rangeEnd   = $rangeStart->endOfMonth(); // Mutates $rangeStart too!

$query = Order::whereBetween('created_at', [$rangeStart, $rangeEnd]);

// ✅ With CarbonImmutable — safe and clear
$rangeStart = now()->startOfMonth();
$rangeEnd   = $rangeStart->endOfMonth(); // $rangeStart unchanged

$query = Order::whereBetween('created_at', [$rangeStart, $rangeEnd]);

When Mutability Is Actually Fine

There are cases where you intentionally want to build up a date step by step. In those situations, mutable Carbon is perfectly acceptable — just be explicit about it:

// Fine — you are deliberately mutating a local variable
$deadline = Carbon::now();
$deadline->addDays(7);
$deadline->setTime(23, 59, 59);

The key rule: if you pass a date to another function or reuse it after modification, use CarbonImmutable.

Choosing the date type

• Switch your Laravel app to CarbonImmutable via Date::use() in AppServiceProvider.
• Use now() and Date::parse() instead of calling Carbon directly.
• Treat immutability as the safe default; reach for mutable Carbon only when you have a deliberate reason.

Related

Laravel coding standards covers the wider project conventions.