Let's debug...

Let's break your predicate first:

    let afterPredicate = NSPredicate(format: "timestamp >= %f, startTimeStamp)
    let beforePredicate = NSPredicate(format: "timestamp < %f", endTimeStamp)
    let combinedPredicate = NSCompoundPredicate(andPredicateWithSubpredicates: [afterPredicate, beforePredicate])


With:

    func executeRequest(predicate: NSPredicate) -> [Item] {
        let request = NSFetchRequest<Item>(entityName: "Item")
        request.predicate = predicate
        var items: [Item] = []
        context.performAndWait {
            do {
                let fetched = try context.fetch(request)
                items.append(contentsOf: fetched)
            } catch {
                print(error)
            }
        }
        return items
    }


Now, let's test it:

    let after = executeRequest(predicate: afterPredicate)
    print("After: \(after)")
    let before = executeRequest(predicate: beforePredicate)
    print("Before: \(before)")

    let combined = executeRequest(predicate: combinedPredicate)
    print("Combined: \(combined)")

    let all = executeRequest(predicate: NSPredicate(value: true))

    let filterAfter = all.filter { afterPredicate.evaluate(with: $0) }
    print("FilterAfter: \(filterAfter)")
    let filterBefore = all.filter { beforePredicate.evaluate(with: $0) }
    print("FilterBefore: \(filterBefore)")
    let filterCombined = all.filter { predicate.evaluate(with: $0) }
    print("FilterCombined: \(filterCombinedAfter)")