This is a little hard to answer without knowing what you are specifically trying to do, because the topic is so broad.
The different date functions available in Crystal are visible in the formula editor. Just open the Date/Time, Date Ranges, and Type Conversion folders in the Functions tree that should appear in the top middle portion of the formula editor.
You can also search the help for "Date", "Date Range functions", and "Date/Time functions". Basic addition and subtraction of dates are also possible, and you can convert the Date field to other formats as well.