If you need to calculate a time difference between your Web server and visitors to your site, look into the JavaScript getTimezoneOffset() method. It returns the time zone difference in minutes between the local time on the client machine and Greenwich Mean Time (GMT).
This script (utime.asp) uses Active Server Pages (ASP) and JavaScript to put the browser’s current time into a hidden form field. When the user clicks OK, your server can read the offset value.
"" then Response.Write "The time offset in minutes is: " & request("thetime") & "
"else%>Click OK to tell us the time offset.
Once you have the user’s offset, you can calculate the time zone difference from your server.