Same day number, six months on
Adding six months keeps the day of the month and advances the month by six, rolling into the next year when needed. This "corresponding day" method is the legal and business default, and it is how the calculator works.
Six months in other units
- Days. Six calendar months are 181 to 184 days. The shortest spans start in a stretch with February; the longest run through several 31-day months.
- The half-year figures. 182 days is exactly 26 weeks and the usual "half a year" approximation; 183 days is half of a leap year. Neither equals six calendar months exactly.
- Not 180 days. Six months is never exactly 180. In a legal deadline that 1-to-4-day gap can be the difference between meeting and missing it, so match the unit the rule actually uses.
The end-of-month rule
The only wrinkle is a start date near the end of a long month. If the target month has fewer days, the result clamps to that month's last day rather than spilling into the next month.
| Start day | Target month has | Result day |
|---|---|---|
| 31st | 30 days | 30th (clamped) |
| 31st | February, 28 days | 28th (clamped) |
| 29th, 30th or 31st | February, leap year | 29th |
| 15th | any month | 15th (no clamp) |
Many date libraries, including JavaScript's own Date, get this wrong and overflow into the next month, so a hand calculation needs the clamp applied deliberately.
Where a six-month deadline shows up
- Passport validity. Over 70 countries require your passport to stay valid at least six months from your date of entry, not your departure. Missing it by days can mean being turned away at boarding.
- Auto insurance. Most major US insurers write six-month policies rather than annual ones, so premiums reset twice a year.
- Probation and check-ups. Three to six months is the typical job probation window, and six months is the default interval for dental and many medical follow-ups.
Common questions
Is six months the same as 180 days?
No. Six calendar months run from 181 to 184 days depending on which months they span, never exactly 180. The common half-year approximation is 182 days (26 weeks). If a contract says "180 days," count 180 days, not six months.
How is six months from a date calculated?
You keep the same day number and move six months forward on the calendar. The one exception is the end of the month: if the start day does not exist in the target month, it clamps back to that month's last day.
What happens to the 31st when I add six months?
It clamps to the last day of the target month. Adding six months to the 31st of a month landing on a 30-day month gives the 30th, and landing on February gives the 28th or 29th. This is the standard "corresponding day" rule courts use.
