With the move to the web-based version of their software, the YNAB folk shook up their core 4 Rules and replaced “Live on Last Month’s Income” (aka The Buffer) with the Age Of Money [AOM] metric. The AOM looks backward in time offering a general view of how long a dollar sits in your budget before it gets spent.
With the Days of Buffering Metric [DOB] setting, the Toolkit for YNAB developers have attempted to bring back some of the missing emphasis on building a buffer. The DOB metric is meant to calculate, roughly, how long your existing funds would last at your current daily average spending rate.
As you can see in my screenshots below, it’s possible to have a negative buffer if you’re in debt (below, left). [My demo budget is a bunch of randomly made up numbers and backdated transactions so the AOM and DOB numbers aren’t necessarily representative of a real-life situation.]
This setting can be used in combination with the Days of Buffering History Lookup setting also in the Budget Screen Settings of the Toolkit for YNAB.