src/precore/util/Stopwatch.php
public function start() : Stopwatch
    {
        Preconditions::checkState(!$this->isRunning, 'This stopwatch is already running.');
        $this->isRunning = true;
        $this->startTick = $this->ticker->read();
        return $this;
    }
public function start() : Stopwatch
    {
        Preconditions::checkState(!$this->isRunning, 'This stopwatch is already running.');
        $this->isRunning = true;
        $this->startTick = $this->ticker->read();
        return $this;
    }
Starts the stopwatch.

@return $this
@throws IllegalStateException if this Stopwatch is already running