Detecting visitors country using Javascript

About

Whilst trying to work out how to show location based ads on my site I thought of a way to do it using just Javascript. The idea is to detect the location based on their computers time different to GMT.

Implementation

The following code will detect if a visitor is from America:

<script> var now = new Date(); var off = (now.getTimezoneOffset()); if (off == 240 | off == 300 | off ==360 | off ==400) { document.write("American ad code"); } </script>

Further Info

The following image displays the different time zones: