Difference between TODAY() and NOW() in Salesforce?

If TODAY() Doesn’t work try using NOW() instead.
The difference is NOW() returns a datetime while TODAY() only returns a date.
You should convert before using both, like below
DATEVALUE(Due_Date) – TODAY().

Ex:
TODAY() – 2013-02-10
NOW() –  2013-02-10 01:30:45 AM