Why September Is the 9th Month When "Sept" Means Seven

Four of our month names are a numbering puzzle hiding in plain sight. Their Latin roots are unambiguous:

• September — septem, seven
• October — octo, eight (think octopus, octagon)
• November — novem, nine
• December — decem, ten (think decimal, decade)

Yet September is the 9th month, not the 7th. Every one of them sits exactly two positions later than its name claims. That is not a coincidence or a translation error — it is a fossil left behind when the calendar was rebuilt.

The Roman year started in March

The earliest Roman calendar began the year in March, not January. Count from there and the names snap into place:

Position	Month	Name means
1	March	(Mars)
…	…	…
7	September	seven ✓
8	October	eight ✓
9	November	nine ✓
10	December	ten ✓

A March start made sense: it lined up with the spring equinox and the start of the military campaign and farming season. The numbered months were simply counted off from there, and back then they were the last four months of the year.

The winter that didn’t count

That original calendar had only ten months and about 304 days. The roughly 60 days of deep winter — a stretch with no harvest and no campaigning — weren’t assigned to any month at all. They were just an unnamed gap until spring.

Eventually two months were added to fill it: January and February, tacked on to cover winter. Now the year had twelve months, but the numbered names still counted from the old March starting line.

January steals the front of the line

The final shift was moving the start of the year to January 1. This happened in Rome for administrative and political reasons (newly elected consuls began their term in January) and was locked in by later reforms.

The moment January became month #1, everything downstream shifted by two:

• September, the old 7th month, was shoved into the 9th slot.
• December, the old 10th month, became the 12th.

Nobody renamed them. Changing four month names across an entire empire’s records was far more trouble than just living with the mismatch — so the numbers froze in place, permanently two off.

The months that honor emperors

Two other month names have their own backstory. July and August were originally called Quintilis (fifth) and Sextilis (sixth) — more numbered months from the March-start era. They were renamed to honor Julius Caesar and Augustus, the two figures behind the Roman calendar reforms.

So of our twelve months, the naming scheme is a patchwork: gods (March, June), rituals (February), an emperor or two (July, August), and four stranded numbers that no longer match their position.

Counting months today

The irregular history is a big reason months are awkward to compute with — they don’t have a fixed length, so “how many months between two dates” isn’t a clean division. A month averages 30.44 days, but any individual one runs 28 to 31.

The months converter uses that average for unit math, while Months Between Dates counts actual calendar months between two dates, respecting their real lengths.

Key takeaways

• September–December mean 7–10 because the Roman year originally started in March.
• The calendar first had only 10 months, ignoring winter entirely.
• Adding January and February, then moving the year’s start to January, pushed the numbered names two slots later.
• The names were never corrected, so the off-by-two mismatch has survived for over two millennia.