Initially the client system will download all relevant records, so the <lastSync> will not be set.
Bundle and get the latest
To prevent missing records, synchronise your client system with the PCA server clock when setting bookmarks.
The safest way to do that is to traverse through the bundle and get the latest _lastUpdated field of a resource in the bundle
if (lastSync < read.resource.meta.lastUpdated) set lastSync = meta.lastUpdated
To select a service offering between a specific date range, use the _lastUpdated field with greater than and less than operators as shown in the example below: