Firstly, understand that DateTime objects aren't formatted. They just store the Year, Month, Day, Hour, Minute, Second, etc as a numeric value and the formatting occurs when you want to represent it as a string somehow. You can compare DateTime objects without formatting them.
To compare an input date with DateTime.Now, you need to first parse the input into a date and then compare just the Year/Month/Day portions:
DateTime inputDate;
if(!DateTime.TryParse(inputString, out inputDate))
throw new ArgumentException("Input string not in the correct format.");
if(inputDate.Date == DateTime.Now.Date) {
// Same date!
}