Citizenship application problem

[derbar]

I am a greencard holder for over 10 years & now intend to apply for citizenship, my problem is that I am unsure of some of the dates that I spent outside the US, The last 5 years are fine, I have a record of those dates but it is the first 5 years that I have a problem with, I have all my return dates which are stamped in my Passport but cannot tell exactly when I left, I could guess but don't want to risk getting it wrong. I did write to the "US Customs & Border Protection" for help but they denied my request.
Any advice is welcome. Thanking you in advance.
Regards...........Dermot.