I recommend looking deeper into DateTime. Here are some pointers:

• QandASection: dates and times in our Q&A section.
• How do I find the difference in days between two dates, in a cool perl way?
• Comparing two dates and getting back difference in days

See also How do I post a question effectively?. It would be helpful to know what format your dates are in now. Regardless of what you do, probably most of the work will be in getting the dates you have into a format that makes them easy to compare, so the current format is very important to answering your question.