The readme says that "immutableMinTimeToLive is a number of milliseconds", but it's compared against values which are in seconds (e.g. Math.max(defaultMinTtl, (expires - dateValue) / 1000)) and returned from maxAge which on other paths returns values in seconds.

In my opinion, the implementation is reasonable so it's the documentation which is wrong; what do you think?

The default value is 24 * 3600 * 1000, which is either one day or 1,000 days depending on the answer to this question. I don't see anything in the RFC guiding how clients should behave if max-age is not set but immutable is. Since "hours * seconds * milliseconds" order makes a little more sense than "hours * seconds * days", I'm guessing it was supposed to be one day, in which case the default value is wrong too.

While you're looking at the documentation for this option:

• The link to "per RFC" is dead. I believe at this point you want to link to https://tools.ietf.org/html/rfc8246.
• Please document the default value for this option, whatever you decide it should have been.