Custom JSON Unmarshalling in Go

I’m working on a consumer for GitHub’s Organization Webhooks, and ran into an issue today where the repository.pushed_at field is sent as a UNIX epoch timestamp in one event time, and an RFC 3339 / ISO 8601 time in another. Naturally, parsing this in Go, both uint64 and time.Time will fail sometimes.

I solved this by implementing a new type, UnixOrRFC3339Time, which implements the UnmarshalJSON method:

// UnixOrRFC3339Time works around an inconsistency in the GitHub API where the
// "pushed_at" field is sent alternately as an integer or RFC3339 (ISO 8601)
// timestamp.
type UnixOrRFC3339Time struct {
    t time.Time
}

// Time returns the parsed time.Time object after loading the JSON.
func (u *UnixOrRFC3339Time) Time() time.Time {
    return u.t
}

// UnmarshalJSON loads either an RFC3339-formatted time (e.g.
// '"2017-11-03T15:15:09Z"') or unix epoch timestamp (e.g. '1498827360').
// Yes, it *is* unfortunate that we need to do this.
func (u *UnixOrRFC3339Time) UnmarshalJSON(data []byte) error {
    if err := u.t.UnmarshalJSON(data); err == nil {
        return nil
    }

    secs, err := strconv.ParseInt(string(data), 10, 64)
    if err != nil {
        return err
    }
    u.t = time.Unix(secs, 0)
    return nil
}

When I give this as the type in the struct I feed to json.Unmarshal, I get a properly parsed time with either message format. It would be great to be able to just declare this type as an alias of time.Time with a custom UnmarshalJSON implementation, but I don’t think I can do that without access to time.Time’s private members.