protected Date getReferenceDate() throws IOException {
        Date referenceDate = EnvUtil.loadTimestamp(getBuildTimestampFile());
        return referenceDate != null ? referenceDate : new Date();
    }
Get the current build timestamp. this has either already been created by a previous
call or a new current date is created
@return timestamp to use
src/main/java/io/fabric8/maven/docker/AbstractDockerMojo.java