One of the consequences of changes in the shape of Earth's orbit over time (in addition to pacing glacial–interglacial variations) is changes in the length of months or seasons. The well-known paleo calendar effect that results can produce patterns in comparisons of present-day and paleoclimate model simulations that could be mistaken for real climate changes. We illustrate the source of those patterns and describe an approach and set of programs for routinely adjusting for the effect.