Why are you opening and closing the file inside your loop? Open it once outside your loop, close it once when the loop finishes. Also, before the loop starts (i.e. outside the loop right after you open the file) write the date and time, then leave it out of your records inside the loop.
Furthermore, I believe the only reason you need to keep setting the file pointer to "end" is because you keep opening and closing the file. Just leave the file open and the file pointer will more as you write; no need to set it before every single file write.