Getting values out of DynamoDB

Is there an effective way for dynamo db to give my web app all the entries created between two dates?

Yup, please have a look at the of the Primary Key concept within Amazon DynamoDB Data Model, specifically the Hash and Range Type Primary Key:

In this case, the primary key is made of two attributes. The first attributes is the hash attribute and the second one is the range attribute. Amazon DynamoDB builds an unordered hash index on the hash primary key attribute and a sorted range index on the range primary key attribute. [...]

The listed samples feature your use case exactly, namely the Reply ( Id, ReplyDateTime, ... ) table facilitates a primary key of type Hash and Range with a hash attribute Id and a range attribute ReplyDateTime.

You'll use this via the Query API, see RangeKeyCondition for details and Querying Tables in Amazon DynamoDB for respective examples.

can dynamo db give me all entries in which a field matches a certain number. [...] Can I ask dynamodb to give me all the entries which have value e.g. 6?

This is possible as well, albeit by means of the Scan API only (i.e. requires to read every item in the table indeed), see ScanFilter for details and Scanning Tables in Amazon DynamoDB for respective examples.

Do both of these queries need a scan of the entire dataset (which I assume is a problem given the dataset size?)

As mentioned the first approach works with a Query while the second requires a Scan, and Generally, a query operation is more efficient than a scan operation - this is a good advise to get started, though the details are more complex and depend on your use case, see section Scan and Query Performance within the Query and Scan in Amazon DynamoDB overview:

For quicker response times, design your tables in a way that can use the Query, Get, or BatchGetItem APIs, instead. Or, design your application to use scan operations in a way that minimizes the impact on your table's request rate. For more information, see Provisioned Throughput Guidelines in Amazon DynamoDB.

So, as usual when applying NoSQL solutions, you might need to adjust your architecture to accommodate these constraints.