you use javascript to add the date to a form post.
if the controller action is a form post, add a hidden field and set it value with javascript:
document.getElementById("fieldId).value = (new Date()).toISOString();
if the controller action is a get, render a sniffer html page that has a small form with the data required, the snippet above and does a:
document.getElementsByTagName[0].submit();
note: you could also use javascript to get the timezone.