Using javascript's date.toISOString and ignore timezone

I want to use the javascript's toISOString() function and ignore the timezone.

var someDate; // contains "Tue May 26 2015 14:00:00 GMT+0100 (Hora de Verão de GMT)"
dateIWant = someDate.toISOString(); // turns out "2015-05-26T13:00:00.000Z"

The date to convert is Tue May 26 2015 14:00:00 GMT+0100 (Hora de Verão de GMT) but the converted date is 2015-05-26T13:00:00.000Z.

So, I need the date in the yyyy-MM-ddTHH:mm:ss:msZ but as you can see above it applies the timezone and changes the hour from 14 to 13.

How to achieve this?