visited-file-modtime

visited-file-modtime is a built-in function in `C source code'.

(visited-file-modtime)

Return the current buffer's recorded visited file modification time.
The value is a list of the form (HIGH LOW USEC PSEC), like the time values that
`file-attributes' returns. If the current buffer has no recorded file
modification time, this function returns 0. If the visited file
doesn't exist, return -1.
See Info node `(elisp)Modification Time' for more details.