This would be very nice to prevent unnecessary requests to the Trakt API, specially since they're imposing limits now. Here's how I think it should happen:

1. Upon opening the sync page for a streaming service for the first time, the user is prompted with an option to sync their entire history so far (since the Trakt API allows 1000 requests every 5 minutes, it's perfect for scenarios like this)
2. We store the date of this initial sync (let's call it date A)
3. In the future, every time the user opens the sync page for that same streaming service again, we make sure that only history newer than date A is loaded
4. Additionally, every time we detect that the user has synced all history from date A to a date B, we update date A with date B

Not only would this save a ton of requests, but I believe it would also enhance the user experience, since they would only see items that they haven't synced. What do you think?