How to select a partition key for for a DynamoDB query?
To query your table using CreatedDate without knowing the ItemId, you can use Global Secondary Index write sharding by adding an attribute (e.g., ShardId) containing a (0-N) value to every item that you will use for the global secondary index partition key.
Depending on how your items are distributed against CreatedDate, you can set the ShardId so that it is likely to have evenly distributed access patterns. For example: YYYY, YYYYMM or YYYYMMDD. Then, you create a global secondary index with ShardId as an index partition key and CreatedDate as an index sort key.
Knowing the primary key for your GSI (since the ShardId value is derived from CreatedDate), you can query the table for the 100 most recent items with query's Limit parameter (or LastEvaluatedKey if your items set size is larger than 1 MB of data).
See Using Global Secondary Index Write Sharding for Selective Table Queries.