Oracle (at time of writing) doesn’t support an PLSQLAssociativeArray of type OracleDbType.TimeStamp.
So what you need is to use a standard string instead then convert it to a timestamp. For this you will need to set the timestamp format via:
ALTER SESSION SET NLS_TIMESTAMP_FORMAT = 'DD/MM/YYYY HH24:MI:SS'
Then in your OracleCommand CommandText you can use:
where t is an input OracleParameter of type OracleDbType.Varchar2 with CollectionType of OracleCollectionType.PLSQLAssociativeArray and the values are formatted as per the NLS_TIMESTAMP_FORMAT.