What is the difference between an undated planner and a bullet journal?

An undated planner ships with pre-printed weekly or monthly layouts but no fixed dates, so you fill in the calendar yourself and can start any time of year. A bullet journal (often a dot-grid notebook) has no layouts at all, giving you a blank dotted page to design your own spreads. Several picks here straddle both, dot-grid notebooks marketed for bullet journaling alongside structured undated planners like the Clever Fox.