fs: document the Date conversion in Stats objects

Document why the dates are calculated with the timestamp
in Numbers + 0.5.

The comment was previously lost in a revert.

Refs: ae6c704

PR-URL: #28224
Refs: ae6c704
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

lib/internal/fs/utils.js

@@ -287,6 +287,12 @@ function nsFromTimeSpecBigInt(sec, nsec) {
   return sec * kNsPerSecBigInt + nsec;
 }
 
+// The Date constructor performs Math.floor() to the timestamp.
+// https://www.ecma-international.org/ecma-262/#sec-timeclip
+// Since there may be a precision loss when the timestamp is
+// converted to a floating point number, we manually round
+// the timestamp here before passing it to Date().
+// Refs: https://github.com/nodejs/node/pull/12607
 function dateFromMs(ms) {
   return new Date(Number(ms) + 0.5);
 }