file-write-date

(file-write-date pathname)

The file's last-modification time as a universal time (seconds since 1900-01-01 GMT), or nil when it cannot be determined — which is what a missing or unreadable file answers. Like probe-file it never signals, so it can be used as a probe. The path is interpreted exactly as open interprets it.

Both WASM backends always answer nil: the timestamp is not lifted there. Unlike file-length, which stats a descriptor already in hand, this one names a PATH, so it would need an open/stat/close of its own. nil is precisely Common Lisp's answer for "the time cannot be determined", so a portable caller's unknown-time fallback runs rather than the program failing. The interpreter and the JVM answer for real.

(let ((stamp (file-write-date "config.lisp")))
  (if stamp
      (print (decode-universal-time stamp))
      (print "unknown")))