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Apparently the DateInterval class supports milliseconds, but the default method does not support it as an input value. You need to instead use the createFromDateString class of that method // convert your date to DateTime object $date = '10:00:00.500000'; $dt = new DateTime($date); // convert your period to $interval = '00:25:10.300000'; //Extract time parts list($hours, $minutes, $totalSeconds) = explode(':', $interval); list($wholeSeconds, $milliSeconds) = explode('.', $totalSeconds); //Create interval with milliseconds $intervalString = "{$hours} hours + {$minutes} minutes + {$wholeSeconds} seconds + {$milliSeconds} microseconds"; $interval = DateInterval::createFromDateString($intervalString); // Add interval to date $dt->add($interval);// Format date as you needecho $dt->format('H:i:s'); echo $dt->format('Y-m-d\TH:i:s.u'); //Output: 2021-11-12T10:25:10.800000