DateInterval 怎么减少时间

DateInterval 如何减少时间
比如这个代码是获取当前时间加上3个小时，那么如何获取减少3个小时呢？

PHP code

<!--Code highlighting produced by Actipro CodeHighlighter (freeware)http://www.CodeHighlighter.com/-->(new \DateTime('NOW'))-&gt;add(new \DateInterval('PT3H'))

------解决方案--------------------
DateInterval::format() - Formats the interval
DateTime::add() - Adds an amount of days, months, years, hours, minutes and seconds to a DateTime object
DateTime::sub() - Subtracts an amount of days, months, years, hours, minutes and seconds from a DateTime object