While a comment from Gayot mostly explained it, I thought I'd expand and add my experience.
For various other visas, I've also had to explain my past travels. I travel a LOT (more so a few years ago, but still try), and as you've found, not every passport stamp is clear, or visible, and frankly, some countries don't even stamp!
If you use gmail, or another email service where you archive your emails, I found this invaluable for searching - I'd type in a country name into my inbox search and often discussions, chats, emails or bookings would show up, indicating this.
As a geek, I also run Google Latitude (now part of Google+) which tracks my location history - I've been able to use that to work out where I was (or when I was on day x).
Another option if you've been at a company for a while is to ask HR - they may have a record of your leave in the past x years, and that may also help narrow it down.
Finally, as Gayot said, add a note explaining that the dates are a best effort guess, from all those sources. The countries are often more important than the dates, in my experience.