How to prove a file was not tampered?
I have uploaded the assignment on website before and I didn't realize my file is not exist on the website until I found out that my assignment wasn't graded. My lecturer asked me to prove the date of my file which is my assignment is before the deadline and never be tampered. I have the source files that show timestamps that are clearly before the deadline. How to prove the date is reliable?
PS: I sent my assignment file which is as same as my source file to my classmate's laptop by a chat software in order to check the file was correct, and there is still the date of the file in the chat software history list and i can still use the software of his laptop to receive and open the file again in his laptop.