You'll need to do a retrieval of the timestamps/values in this time range in order to do a delete. There is an enhancement request available for this feature on our feedback site here: Add bulk delete methods in the Web API – Customer Feedback for OSIsoft & the PI System
I'll see if I can come up with a batch request that can serve as a workaround.
Edit: It is currently not possible to do this in a batch request due the inability to add parameters as the body. See this enhancement for info on that: Allow parameters to be passed to a request body in the Batch Controller – Customer Feedback for OSIsoft & the PI System