public String putDateTime(final String key, final Instant instant) {
        if (instant == null) {
            return super.put(key, null);
        } else {
            final String dateStr = DateUtil.createNormalDateTimeString(instant.toEpochMilli());
            // Already normalised, so use super.put not the local one
            return super.put(key, dateStr);
        }
    }
Puts the specified {@link Instant} 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