private DeploymentSpec.ChangeBlocker readChangeBlocker(Element tag) {
        boolean blockVersions = trueOrMissing(tag.getAttribute("version"));
        boolean blockRevisions = trueOrMissing(tag.getAttribute("revision"));

        String daySpec = tag.getAttribute("days");
        String hourSpec = tag.getAttribute("hours");
        String zoneSpec = tag.getAttribute("time-zone");
        String dateStart = tag.getAttribute("from-date");
        String dateEnd = tag.getAttribute("to-date");

        return new DeploymentSpec.ChangeBlocker(blockRevisions, blockVersions,
                                                TimeWindow.from(daySpec, hourSpec, zoneSpec, dateStart, dateEnd));
    }
Returns the given non-blank attribute of tag or throw
config-model-api/src/main/java/com/yahoo/config/application/api/xml/DeploymentSpecXmlReader.java