The system lets you create a budget that has in essence expired, but if you try to add funds to it those funds aren't added and no error appears explaining why not. If you can't have a fund in the past then you shouldn't be able to have a budget in the past - and in both cases there should be some sort of warning or error to the user so they know what's happening.
I retested on current master and adding funds to inactive budgets with dates in the past works now.