We can use Tamper Data for tampering the POST requests, but I want to know if there is any tool to tamper the requests which are mix of GET and POST
like : http://example.com/?time=123456&date=22-12-2012
and also have associated POST data.
With Tamper Data I can only change the POST parameters and not GET, and if I use another way to change GET parameters, I loses the POST data.
Try fiddler. That gives you control over GET and POST data (the entire request in general) and should be able to do what you desire.
It even has its own associated FiddlerScript which helps automate handling (redirection, editing, etc) of web requests.