The FILE_MODTIME function returns the modification time of the specified file or files.
            Examples
            To retrieve the modification time of dist.pro within the IDL library:
            
            mtime = FILE_MODTIME(FILEPATH('dist.pro', SUBDIR = 'lib'))
            
            PRINT, SYSTIME(0, mtime)
            IDL prints:
            Thu Sep 24 10:48:19 2015
            Syntax
            Result = FILE_MODTIME(File) 
            Return Value
            The result is a scalar or array of type LONG64. If File is a scalar then the result is a scalar, otherwise the result is an array of the same dimensions as File.
            Arguments
            File
            A string or string array containing the path(s) to the file(s).
            Keywords
            None.
            Version History
            
            See Also
            FILE_INFO,  FSTAT