Mastering Timezone Differences in PHP: Why strtotime Trumps diffInSeconds for Absolute Seconds

Dealing with timezones and calculating accurate differences in seconds is a frequent source of confusion in application development, especially when working with libraries like Carbon. As senior developers, we often encounter scenarios where methods that seem logically sound yield unexpected results. Today, we will dissect the difference between using Carbon's built-in time difference methods versus leveraging PHP’s native strtotime() function to find the true, absolute difference in seconds across different timezones.

The Timezone Conundrum: Local vs. Absolute Time

The core of your question revolves around this discrepancy: why does Carbon::diffInSeconds seem to fail when you expect a simple subtraction, while strtotime() provides what appears to be the perfect answer? This difference stems from how each method handles timezone context versus raw epoch time.

When you work with objects like Carbon instances, they are inherently aware of their specific timezone (e.g., 'Australia/Perth'). When you use methods like diffInSeconds(), Carbon calculates the difference between two points in time, factoring in the timezone offsets to determine the elapsed duration as displayed locally. However, when dealing with absolute differences across disparate systems or arbitrary strings, we often need a reference point that ignores the local display entirely.

Deconstructing the Code Example

Let’s examine the logic you presented:

    echo $tz = Carbon::now('Australia/Perth'); // Current time in Perth context
    echo "<br>";
    $local='2017-04-11 12:39:50';
    echo $emitted = Carbon::parse($local); // The specific historical time
    echo "<br>";
    echo "diff from carbon->";
    echo $diff = $tz->diffInSeconds($emitted); 
    echo "<br> diff from Normal->";
    echo  $diff1 = strtotime($tz) - strtotime($emitted);

The result you observed—where strtotime() provided the expected difference and diffInSeconds() seemed off by two hours—is entirely predictable based on how these functions operate.

The Role of diffInSeconds

The diffInSeconds() method calculates the time delta between two Carbon objects. While powerful for duration calculations within a localized context, it relies on the internal representation of those instances. If the timezone difference shifts the perceived local time boundary, the calculated time difference might reflect that shift rather than the raw chronological gap in seconds.

The Power of strtotime() and Epoch Time

The function strtotime() is designed to parse a human-readable date/time string and convert it directly into a Unix timestamp (the number of seconds since January 1, 1970, UTC). When you subtract two timestamps generated by strtotime(), you are performing a direct subtraction on the absolute epoch time. This method bypasses complex timezone interpretation for the difference calculation, yielding the exact chronological difference in seconds between those two points, regardless of which timezone they were initially defined in.

Best Practice: Working with UTC for Reliability

For any serious application involving time comparisons, especially when dealing with APIs, databases, or distributed systems—a core concept in modern Laravel applications—the best practice is to standardize everything to UTC before performing calculations. This eliminates ambiguity caused by local timezone offsets.

Instead of relying on potentially misleading local differences, convert both times to UTC timestamps first:

use Carbon\Carbon;

// 1. Define the reference time in a specific timezone (e.g., Sydney)
$sydneyTime = Carbon::now('Australia/Sydney');

// 2. Parse the target time string and ensure it's interpreted correctly
$targetLocal = '2017-04-11 12:39:50';
$targetCarbon = Carbon::parse($targetLocal, 'Australia/Perth'); // Parse with target zone

// 3. Convert both to UTC for a reliable difference calculation
$sydneyUtc = $sydneyTime->utc();
$targetUtc = $targetCarbon->utc();

// Calculate the true absolute difference in seconds
$absoluteDifference = $sydneyUtc->diffInSeconds($targetUtc);

echo "Absolute Difference (UTC): " . $absoluteDifference; 

By converting both points to UTC, you ensure