How send C# DateTime object into Javascript?

I don't know which method you use to "send" the value to JavaScript (include, HTTP response,...), but I usually format the DateTime to yyyy/MM/dd HH:mm:ss, pass it to the client, then I parse it with JavaScript using new Date(Date.parse({the date string}), which allows me to format it the way I want on the client-side, using the locale or other formatting constraints.

Pretty much trivial:

<script type="text/javascript">
var strServerTime = "<%=DateTime.Now%>";
alert("server time: " + strServerTime);
</script>

However, what you want to do with that date? If only to display, the above should suffice otherwise explain and we'll see how to proceed.

Convert it to a suitable string, and if you need to, have javascript parse it.

var myDate = DateTime.Now;
            Page.ClientScript.RegisterClientScriptBlock(this.GetType(), "Date", 
                String.Format(
                "var myDate = new Date({0}, {1}, {2}, {3}, {4}, {5}, {6});",
                myDate.Year,
                myDate.Month,
                myDate.Day,
                myDate.Hour,
                myDate.Minute,
                myDate.Second,
                myDate.Millisecond
                ), true);

Or

var javaBaseDate = new DateTime(1970, 1, 1);
var myDate = DateTime.Now;
Page.ClientScript.RegisterClientScriptBlock(this.GetType(), "Date", 
  String.Format(
  System.Globalization.CultureInfo.InvariantCulture,
  "var myDate = new Date({0});",
   (myDate - javaBaseDate).TotalMilliseconds
   ), true);