C#

void textBox1_TextChanged(object sender, EventArgs e)
{
... //Check the input
textBox1.TextChanged -= new EventHandler(textBox1_TextChanged);
... //Change the text
textBox1.TextChanged += new EventHandler(textBox1_TextChanged);
}

//quick and dirty, this won't work as entered, but is to provide concepts
if (entryisvalid())
{
   textbox.color=color.black;
   textbox.text = doformatting(textbox.text);
}
else
{
   textbox.color=color.red;
   messagebox.show("your input is invalid, please correct the entry in red");
}

DateTime.Now.ToString("ddMMyyyyHHmm") + ".text"