you can get only the date from ASP.NET Calendar control. you need to find jQuery or other extended Calendar control which support both Date and Time, Check below Article for how to use jQuery DateTime picker and fetch selected datetime from code behind.
DateTimePicker control for ASP.Net TextBox Example[
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Then you can insert selected
DateTime
value to a DateTime or DateTime2 column in your SQl server Database like below
DateTime dob = DateTime.Parse(Request.Form[TextBox1.UniqueID]);
cmd.Parameters.AddWithValue("@dob", dob);