code/Debug/Model/Block.php
public function startRendering(Mage_Core_Block_Abstract $block)
    {
        if ($this->isRendering) {
            // Recursive block instances with same name is used - we don't update render start time
            $this->renderedCount++;

            Mage::log("Recursive block rendering {$this->getName()}", Zend_Log::DEBUG);
            return;
        }

        // Re-init data from block (some extension might update dynamically block's template)
        $this->init($block);

        $this->isRendering = true;
        $this->renderedCount++;
        $this->renderedAt = microtime(true);

        if (self::$startRenderingTime===null) {
            self::$startRenderingTime = $this->renderedAt;
        }
    }
public function startRendering(Mage_Core_Block_Abstract $block)
    {
        if ($this->isRendering) {
            // Recursive block instances with same name is used - we don't update render start time
            $this->renderedCount++;

            Mage::log("Recursive block rendering {$this->getName()}", Zend_Log::DEBUG);
            return;
        }

        // Re-init data from block (some extension might update dynamically block's template)
        $this->init($block);

        $this->isRendering = true;
        $this->renderedCount++;
        $this->renderedAt = microtime(true);

        if (self::$startRenderingTime===null) {
            self::$startRenderingTime = $this->renderedAt;
        }
    }
Captures rendering start for specified block

@param Mage_Core_Block_Abstract $block
@throws Exception