Is the `mtime` property of the `stats` object provided by `fs.stat` always a valid `http-date`?

I'm using fs.stats to get the modified time (mtime) of a file so that I can put that information into a last-modified header when I serve that file. Is this property guaranteed to provide me with a valid http-date every time?

In other words, will this property always give me something which I can safely pass into a last-modified header?