How to marshal an x509.Certificate object to bytes in Go

Question

I have an x509 certificate. As part of my testing, I want to adjust the "NotBefore" and "NotAfter" fields in this certificate to test various edge cases in my code. I was hoping that I could read this certificate into my Go program, change the fields, then write it back to disk.

However, it seems like there isn't a way to marshal an x509.Certificate object back to bytes in Go. Am I just missing the function for this? Or maybe there is something I'm not understanding about DER format.

func main() {
    fpath := "/my/certificate/cert.crt"
    b, err := ioutil.ReadFile(fpath)
    if err != nil {
        panic(err)
    }
    block, _ := pem.Decode(b)
    cert, err := x509.ParseCertificate(block.Bytes)
    if err != nil {
        panic(err)
    }
    cert.NotAfter = time.Now().Add(-24 * time.Hour)

    // How do I marshal cert back to bytes in DER format?
    // b, err := x509.Marshal(cert)
}

The only answer I have found that seems relevant to me is How to get a string out of x509 certificate public key in go? However MarshalPKIXPublicKey doesn't seem correct to me, since it only takes in the public key, instead of the entire certificate.