Can't find mx.formatters.DateBase

Hi,

I have a Flash Builder 4.7 ActionScript project. It's using the  as3corelib. The DateUtil class is getting an error. It can't import mx.formatters.DateBase.

The DateUtil class is here: https://github.com/mikechambers/as3corelib/blob/master/src/com/adobe/u tils/DateUtil.as

Where do I find mx.formatters.DateBase?

Thanks!