Why Months Are the Trickiest Unit in Date Math
Ask what date is one month before March 31 and you hit a problem: February has no 31st.
The end-of-month rule
Most calendars and software clamp to the last valid day. One month before March 31, 2026 is February 28, 2026. One month after January 31, 2024 is February 29, 2024.
Months are not reversible
Go forward one month from January 31 to February 28, then back one month, and you reach January 28, not January 31. That's expected behaviour.
Average month length
For rough conversions, a month averages 30.44 days. For exact dates always move by calendar months, as the months ago calculator does.
Try it in your language
The same tool is available in 20 languages, including Español, Français, Deutsch, 日本語 and العربية. See all calculators.