ISO 8601 Date JS Interpretation Difference - IE/FF versus Chrome

Why do IE/FF and Chrome javascript engines differ on how to interpret this Date format (YYYY-MM-DDTHH:mm:ss.fff) without the timezone designator?

new Date("2015-02-18T15:43:57.803").getUTCHours()

UTC Hours

Chrome: 15

IE11/FF: 21

I don't understand this - is it because Chrome assumes it's local whereas IE/FF assume it's UTC? This seems like a Chrome bug.

Interestingly - appending a "Z" to the end of the string tells both Chrome and IE/FF that the time is UTC and they can agree. Has anyone else noticed this javascript implementation discrepancy with Date?

new Date("2015-02-18T15:43:57.803Z").getUTCHours()

UTC Hours

Chrome: 15

IE11/FF: 15

Ultimately - this is the result of the out-of-box serializer for ASP.NET Web API, which I thought used JSON.NET, but now appears to be internal where JSON.NET uses IsoDateTimeConverter .

Checking GlobalConfiguration.Configuration.Formatters.JsonFormatter tells me we're using JsonMediaTypeFormatter . Is Web API not using JSON.NET serializer out of the box?

This is a boon for Web API people - at least back in ASP.NET MVC we had a consistent date format (albeit proprietary - /Date(number of ticks)/ ) via the JavascriptSerializer