I don't trust printing to the same filehandle you're reading from. That seems like a good way to get in an infinite loop.

In fact, I wouldn't be surprised if it reset the file pointer, subverting your reads.

My approach to your original issue would be to open the file for reading, make a list of new times to add, close it for reading, open it for appending and append all at once. You would avoid the seek and the possible concurrency issues that way. It's really two conceptual steps anyway, and I couldn't get my test program to handle it correctly the other way anyway.