Sep 16, 2026 8:00 AM in response to DrYeet
Because the conversion unit for months is missing from the Time category in the calculator app. Observed in macOS Tahoe 26.7. You might consider submitting a feature request for adding months to themacOS product team.
I keep a link to the WolframAlpha website in my Safari Favorites for one-click access and the following result:

Sep 16, 2026 5:03 PM in response to VikingOSX
Thanks for your help, very strange how it's just "missing" entirely. Apple just has a knack for forgetting things here and there, don't they?