Lost my old passport, stumped by travel history section

I am applying for a Tier 4 General Student visa to the UK. In the form they asked if this was my first passport. I answered no as I have 2 previous passports. Now they want details. I can't seem to locate my second passport and might have left it overseas (if I did, I probably already shredded it to pieces, I can't remember). This problem persist on the travel history question where I need to list all the countries I been to and time. I can remember the countries, but not the time of entry as they are in my old passport.

If you don't know the required details of the lost passport, just say so.

If you don't know the exact dates of your previous travel, and can't get the information anywhere, give your best estimate.