on login button click event
Get current time
Then check this time between 10:00 Am - 10:30 Am
if true then go to authentication else show user friendly message.
Like :
TimeSpan start = new TimeSpan(10, 0, 0);
TimeSpan end = new TimeSpan(10, 30, 0);
TimeSpan now = DateTime.Now.TimeOfDay;
if ((now > start) && (now < end))
{
AuthenticateUser();
}
else
{
}