What's the alternative to IValueFormatter in AutoMapper?

So I started using AutoMapper for my ASP.NET MVC project, looked at a couple of examples and wrote my first IValueFormatter implementation when a blue wiggly popped up for IValueFormatter: Interface 'AutoMapper.IValueFormatter' is obsolete: "Formatters should not not be used". Well, okay, then how am I supposed to e.g. convert a DateTime to, say, a string?