legacy/src/main/java/thredds/crawlabledataset/s3/CrawlableDatasetAmazonS3.java
@Override
    public Date lastModified() {
        S3ObjectSummary objectSummary = objectSummaryCache.getIfPresent(s3uri);
        if (objectSummary != null) {
            return objectSummary.getLastModified();
        }

        ObjectMetadata metadata = threddsS3Client.getObjectMetadata(s3uri);
        if (metadata != null) {
            return metadata.getLastModified();
        } else {
            // "this" may be a collection or non-existent. In both cases, we return null.
            return null;
        }
    }
@Override
    public Date lastModified() {
        S3ObjectSummary objectSummary = objectSummaryCache.getIfPresent(s3uri);
        if (objectSummary != null) {
            return objectSummary.getLastModified();
        }

        ObjectMetadata metadata = threddsS3Client.getObjectMetadata(s3uri);
        if (metadata != null) {
            return metadata.getLastModified();
        } else {
            // "this" may be a collection or non-existent. In both cases, we return null.
            return null;
        }
    }
Returns the date that the dataset was last modified. Will be null if the dataset is a collection or non-existent.

@return the date that the dataset was last modified