The short answer is, search the forums; this gets asked quite regularly.
The medium answer is, why are you storing the date in a varchar?
The longer answer is, read the API docs for SimpleDateFormat, in particular the parse() method. You can create an instance of this class initialized with your date pattern, then use it to parse the dates, creating Date objects. You can then compare these by getting their time in milliseconds, then using simple maths to convert to hours/days/whatever.