The timestamp of the data record from which to start reading. Used with shard iterator type AT_TIMESTAMP. A timestamp is the Unix epoch date with precision in milliseconds. For example, 2016-04-04T19:58:46.480-00:00 or 1459799926.480. If a record with this exact timestamp does not exist, the iterator returned is for the next (later) record. If the timestamp is older than the current trim horizon, the iterator returned is for the oldest untrimmed data record (TRIM_HORIZON).