There is no opening balance formula that allows the user to specify a custom time frame. Currently, you have to choose between referencing last year, quarter, or month. We should be able to do a custom time frame say beginning of time to last month.
And if you apply a filter to show the current period only, opening balances will only show for items that have had activity in the current period. Similar to the "show items with no data" option, there should also be an option to show amounts relating to balances prior to the current filter settings.