How can I convert the date string calculated by CoreData to ISO format YYYY-MM-DD HH:MM:SS +000?

I have a string like this, 285298171.84336197376251220703

and I know it's created by CoreData and I want to find a way to convert it into ISO format, can anyone help me?

Thanks in advance.

Added: actually this is in an XML file which an application created. 285298171.84336197376251220703 开发者_运维技巧281021264.12619298696517944336

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Without more context, it's impossible to be sure, but you're almost certainly observing a string-ified NSTimeInterval (a typedef'd double) that represents the number of seconds since a reference date. You want the uderlying NSDate that it represents which you can convert to any standard string format via NSDateFormatter.