Can Carbon::setTestNow() preserve microseconds when given a float timestamp?

Answer

Carbon 2.x does not preserve microseconds when setTestNow() receives a float; the fractional seconds are truncated. Carbon 3.x is reported to preserve them, but the exact rounding follows PHP's float-to-integer conversion and is not documented in Carbon's release notes. Treat microsecond fidelity as version-dependent until you verify your installed release.

Confirmed facts and version differences

Behavior	Carbon 2.x	Carbon 3.x
Float passed to setTestNow()	Truncated to whole seconds; microseconds zeroed	Fractional part retained, subject to PHP float conversion
format('u') output	Always 000000	Reflects converted microseconds
Effect on CarbonImmutable	Global, process-wide	Global, process-wide

Verify your installation

1. Check the installed major version:

composer show nesbot/carbon

1. Run this scoped script from the project root:

<?php
require 'vendor/autoload.php';
use Carbon\\Carbon;
Carbon::setTestNow(1700000000.123456);
echo Carbon::now()->format('Y-m-d H:i:s.u'), "\n";
Carbon::setTestNow();

Output ending in 000000 indicates 2.x truncation. Output ending in 123456, or a nearby value shifted by float precision, indicates 3.x preservation.

Recommended pattern

Because the clock is global and process-wide, clear it in teardown and avoid raw floats where exact microseconds matter:

$seconds = (int) floor(1700000000.123456);
$micros = (int) round((1700000000.123456 - $seconds) * 1_000_000);
Carbon::setTestNow(Carbon::createFromTimestamp($seconds)->setMicrosecond($micros));
// ... assertions ...
Carbon::setTestNow();

Verify that passing a Carbon instance to setTestNow() behaves identically on your version before adopting this in CI.

Assumptions: PHP 7.1 or newer for Carbon 3.x microsecond support; float timestamps from microtime(true) may already carry rounding artifacts before reaching Carbon.