Perl has several good date/time modules: Date::Manip, Time::CTime, Time::ParseDate

Time::Parsedate can be used to parse many common date formats into a unix timestamp. Time::CTime can be used to format a unix timestamp into just about any date format you want.

Date::Manip is the granddady of all date manipulation modules. It has functions to both parse and format dates in many formats. Date::Manip is large, but will work for dates before 1970 and after 2034 (the limitation of a unix timestamp, and hence the 2 Time:: modules mentioned above).

For extracting the dates from your files you will probably need to examine the files to see what date formats are used (or in what location the occur) and then write a regular-expression to extract them based on format (or location). Once you have them extracted you could use the libraries above to parse and convert to a common date format.