The 'Export from date' setting allows you to control how far back in Synergy that Connect checks for data like:
Invoices (Synergy invoice date)
Credit Notes (Synergy credit note date)
Payments (invoice payment date in either Synergy or Xero)
Bills (Synergy bill date)
Staff expenses (Synergy transaction date for cash and travel staff expenses)
The export date defaults to the first day of the current month when you first open Xero Connect. Update this date to be the date that you will start using Xero Connect from, e.g. the cut over date. Update the date using the calendar or by typing a new export from date.
Then this export from date should be updated to be either the end of the year (calendar or financial) or quarter. This ensures that Synergy doesn't try to lookup and send over data from a past period that is already locked in Xero. The other advantage to rolling this date forward is that the Connect Preview screen loads quicker, as there is less data to check between Synergy and Xero when preview is working out what to display. This is important as the before the preview sync screen is loaded it checks each item against Xero to see if already exists, and then only the items required to be sent are shown in the preview window.