public String putDateTime(final String key, final Long epochMs) {
        final String dateStr = DateUtil.createNormalDateTimeString(epochMs);
        // Already normalised, so use super.put not the local one
        return super.put(key, dateStr);
    }
Puts the specified date time (as epoch millis) into the map in Stroom standard ISO 8601 format,
e.g. {@link DateUtil#createNormalDateTimeString()} using the specified key.

@return The previous value for the key.
stroom-meta/stroom-meta-api/src/main/java/stroom/meta/api/AttributeMap.java