As adamsmith says, better to follow-up on zotero-dev if you have additional questions, but you probably want the `since` query parameter, not If-Modified-Since-Version (which is for conditional requests). https://www.zotero.org/support/dev/web_api/v3/syncing outlines a typical sync process, which does this sort of thing.
https://groups.google.com/forum/#!forum/zotero-dev
, but you have seen the If-Modified-Since-Version:
header option?
https://www.zotero.org/support/dev/web_api/v3/basics#caching