Option 1:
You can use a RegularExpressionValidator control.
Drag and drop a RegularExpressionValidator control on your ASP.NET page next to the control you want to validate and set it's ControlToValidate property to the ID of the TextBox.
Set ValidateExpression property : ^([1-9]|1[0-2]|0[1-9]){1}(:[0-5][0-9][aApP][mM]){1}$
Option 2:
1. Make sure AutoPostBack property of TextBox is True.
2. Write this code on TextChanged event handler of the TextBox.
Protected Sub TextBox1_TextChanged(ByVal sender As Object, ByVal e As System.EventArgs)
Dim t As DateTime
If (DateTime.TryParse(TextBox1.Text, t) = True) Then
Else
' Show your error message here.
TextBox1.Focus()
End If
End Sub