Key facts
- Most months have 30 or 31 days, while February has 28 days in a common year and 29 days in a leap year.
- Seven months have 31 days, four months have 30 days, and February is the exception.
- From March onwards, every month starts one day-number later in a leap year than in a common year.
- The lengths of months make "a month" an imprecise unit for exact day counts.
Month lengths and day of the year
The calendar months vary in length. January, March, May, July, August, October and December have 31 days. April, June, September and November have 30 days. February has 28 days in a common year and 29 days in a leap year. These fixed lengths determine the day number of the year on which each month starts.
| Month | Days | Starts on day (common year) | Starts on day (leap year) |
|---|---|---|---|
| January | 31 | 1 | 1 |
| February | 28 (29 in leap years) | 32 | 32 |
| March | 31 | 60 | 61 |
| April | 30 | 91 | 92 |
| May | 31 | 121 | 122 |
| June | 30 | 152 | 153 |
| July | 31 | 182 | 183 |
| August | 31 | 213 | 214 |
| September | 30 | 244 | 245 |
| October | 31 | 274 | 275 |
| November | 30 | 305 | 306 |
| December | 31 | 335 | 336 |
For example, 1 March is day 60 in a common year such as 2025 but day 61 in a leap year such as 2024, because 29 February comes before it. January and February start on the same day number in every year. To find the total days in a specific month for any year, the days in month calculator can confirm the correct length.
Quarters and half-years
Calendar quarters group months into three-month blocks. Q1 is January to March, Q2 April to June, Q3 July to September, and Q4 October to December. Their total days reflect February's length in leap years.
| Quarter | Months | Days (common year) | Days (leap year) |
|---|---|---|---|
| Q1 | January to March | 90 | 91 |
| Q2 | April to June | 91 | 91 |
| Q3 | July to September | 92 | 92 |
| Q4 | October to December | 92 | 92 |
Half-years also differ slightly. The first half-year (H1) is 181 days in a common year and 182 days in a leap year. The second half-year (H2) is 184 days in both. The quarter calculator shows which quarter a date is in and how many days remain in it, and the date to day number converter gives any date's position in the year.
Remembering which months have 31 days
Two well-known memory aids make this pattern easy to recall. The first is the rhyme: "Thirty days hath September, / April, June and November". All other months have 31 days, except February alone. The second is the knuckle method: make a fist and count the months across the knuckles and the gaps between them, starting with January on the first knuckle. Months that land on a knuckle have 31 days; months that land in a gap have 30, or 28 or 29 for February. July lands on the last knuckle, and you start again from the first knuckle with August, which is why July and August both have 31 days.
Notes on adjacent 31-day months
July and August are both 31 days and follow one another without a 30-day month between them. December and January are also both 31 days, as the year wraps round.
Why month length matters
A month can be 28, 29, 30 or 31 days long. Because of this range, "a month" is an imprecise unit when you need an exact number of days between two dates. For example, adding one month to 31 January gives 28 February in a common year or 29 February in a leap year. This matches the adding months reference, which explains how clamping works for dates that do not exist in the target month.
Similarly, when counting days across month boundaries, the exact lengths matter. DateUtils treats month arithmetic by adjusting to the last valid day of the target month rather than spilling into the following month. For planning date ranges, the start and end date calculator is a practical way to apply these rules to real dates.