[Jtsummers]

YNAB does that. It's one of the things that keeps me on it. My wife is going home to Argentina in October for a month, we've started setting aside an amount of money each month to cover her costs while there (she'll be staying with family so this is money to cover meals/drinks out and things she wants to buy there that aren't easy to get here). The amount builds up to our target for this trip.

The same thing for groceries or any other category. If I budget $300/month normally for groceries, but only spend $200 this month, I have $100 leftover for next month. I can choose to budget the full $300 like normal (have a balance of $400 available) or just $200 to bring it up to my target balance.