CDateTime() converts a string to a date time variable. Just make sure to put a space between your two strings when you concatenate them together and pass it to the function.

I have all the Crystal Reports functions documented with sample code in my book.  Crystal Reports Encyclopedia

First, concatenate them as strings:

{date of fault} & " " & {time of fault}

Then, do your conversion:

CDateTime( {date of fault} & " " & {time of fault} )

Do the same thing with your resolution fields.  Then, they are both datetime fields, and can be compared normally.

The CDate function will convert a datetime value into just a date value.  That's probably the simplest solution.