vim: automatic date/timetamp when opening a specific file?

From vi.stackexchange.com, a seemingly easy answer:

 autocmd VimEnter dairy.md | 0put=strftime('%c') 

You just have to use the filename, the zero indicates the very beginning of the file. If you want to modificate the timestamp just read the strftime help for that.

huge thanks to @romaini and @SergioAraujo~!

@romaini - your comment helped me learn more about external commands and using the built in :help

@SergioAraujo - your answer was right on the money~! the only issue was that I received the following error with your code:

E116: Invalid Arguments for function strftime(

It was a quick fix though by switching from ("%c") to ('%c') :

autocmd VimEnter diary.md | $put=strftime('%c')

I am unsure why it only works with single quotes

Additionally, you will note I used $put instead of 0put, this is because I wanted to add the timestamp at the end of the file as a way to start a new entry of my diary on this document. Once again thank you for all your help~!