Problem Bolding Text Within Textbox?!

could anyone post some code for changing the text to bold within a textbox? im currently making a css class like so

.dateFormat {
font-weight: bold;
}

and then programatically doing the following

alertTextBox.CssClass = "dateFormat";
alertTextBox.Text = System.DateTime.Now.ToString();

but the string is not bold!? what am i doing wrong


 
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