What version of Go are you using (go version)?

$ go version
go version go1.13.6 linux/amd64
$ go list -m golang.org/x/crypto
golang.org/x/crypto v0.0.0-20200109152110-61a87790db17

Does this issue reproduce with the latest release?

Yes

What did you do?

Used a Manager with RenewBefore set to 5 minutes (as a part of a test while running my own boulder instance):

m := autocert.Manager{
    // ...
    RenewBefore: time.Minute * 5,
}

What did you expect to see?

I expected that the manager actually used 5 minutes as RenewBefore, since the documentation states:

// RenewBefore optionally specifies how early certificates should
// be renewed before they expire.
//
// If zero, they're renewed 30 days before expiration.
RenewBefore time.Duration

What did you see instead?

RenewBefore being set to 30 days.

There is a check that makes sure that the value specified is more than the renewJitter (1 hour):
https://github.com/golang/crypto/blob/61a87790db17894570dfb32dbaa0a4af9ce60cb4/acme/autocert/autocert.go#L1098

Either it should be allowed to use values less than one hour, or the documentation should reflect that values less than an hour is the same thing as "30 days".