From when do orders start syncing to Sumtracker?

We start syncing orders after you turn on the inventory sync.We don't sync past orders because the inventory we get from the online platform (at the time of turning on sync) has already taken the past orders into account. So if we get the previous orders into Sumtracker, we risk deducting the inventory again for the same orders.
In order to avoid this confusion, we only sync the new orders.

You may request us to push the date back to sync all orders from a past date. Both open and fulfilled orders will be synced from that date.
But this has a repercussion -
For the past dated closed orders, the inventory will be decreased in Sumtracker for the items in those orders.
For past dated open orders, the inventory will be booked in Sumtracker for all items. 

You'll have to update the inventory back to correct values so that they are updated correctly to Online store again.
You may take the export of inventory values from Sumtracker, that you can use as a backup later.