code: public Date getDocumentExpiration(@NonNull String id) throws CouchbaseLiteException {
          if (id == null) { throw new IllegalArgumentException("document id cannot be null."); }
  
          synchronized (lock) {
              try {
                  if (getC4Database().get(id, true) == null) {
                      throw new CouchbaseLiteException("Document doesn't exist in the database.",
                          CBLError.Domain.CBLITE, CBLError.Code.NOT_FOUND);
                  }
                  final long timestamp = getC4Database().getExpiration(id);
                  return (timestamp == 0) ? null : new Date(timestamp);
              }
              catch (LiteCoreException e) {
                  throw CBLStatus.convertException(e);
              }
          }
      }
docs: Returns the expiration time of the document. null will be returned if there is
  no expiration time set
  
  @param id The ID of the Document
  @return Date a nullable expiration timestamp of the document or null if time not set.
  @throws CouchbaseLiteException Throws an exception if any error occurs during the operation.