How do i see when i changed calendar!

Question

so i can prove that i did wrote in the calendar

Answer 1

Unfortunately you can't.  Only when they edit the file you can, by checking the modification date of the file.
However if you are suspicious of anyone else opening and looking at your files, you can create a folder and call it "Confidential" or something. And then add a Folder Action to that folder to send you an email if anyone opens the folder.
Here is an example of a Folder Action Script you can use if you want to do that:
property theName : "Your Name"
property theAddress : "[email protected]"
on opening folder this_folder
          set {TID, text item delimiters} to {text item delimiters, ", "}
          set this_folder to this_folder as text
          set text item delimiters to TID
          set dateString to (current date) as string
          set theBody to "Someone accessed this folder " & name of (info for this_folder) & "and probably looked at a file…."
          tell application "Mail"
                    set newMessage to make new outgoing message with properties {visible:true, subject:"Someone looked at an item in a your confidential folder on your desktop" & dateString, content:theBody}
                    tell newMessage
  make new to recipient at end of to recipients with properties {name:theName, address:theAddress}
                    end tell
  activate
  send newMessage
          end tell
end opening folder