DateTimeZone exception?

Hi,

I have a piece of code under SugarPro/custom/clients/base/api/Tasks.php

$utcTime = new DateTime("now", new DateTimeZone("UTC"));

it work fine, however when I use the same code in another file SugarPro/custom/modules/Quotes/custom_classes/QuoteNotify.php

it will throw exception, the exception is caused by "new DateTimeZone("UTC")"

The only different I noticed is that below two line is included in QuoteNotify.php

namespace Sugarcrm\Sugarcrm\custom\modules\Quotes\custom_classes;
use DateTime;

I just wonder why the exception error in QuoteNotify.php?

Any suggestion would be appreciated.