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Half birthday calculator: add six months, then fix the end-of-month dates

2026-10-30

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A half birthday calculator does one thing: it adds six months to your birthday. For most dates that is instant. April 25 becomes October 25. November 11 becomes May 11.

The trouble starts at the end of the month. March 31 plus six months should be September 31, and there is no September 31.

A second problem hides behind the first: some tools count days instead of months, so two calculators can give you two different answers for the same birthday.

Below is one rule that settles both, a lookup table for every birth month, and the spreadsheet formula that matches it.

1. When is my half birthday? The simple rule

Keep the day, move the month forward six. That is the whole rule for any birthday on the 1st to the 28th.

• January 27: July 27
• April 25: October 25
• July 4: January 4
• November 11: May 11

▲ For days 1 to 28, the half birthday keeps the same day number

The month pairs never change, so it helps to see them once:

| Birth month | Half birthday month | Birth month | Half birthday month | |---|---|---|---| | January | July | July | January | | February | August | August | February | | March | September | September | March | | April | October | October | April | | May | November | November | May | | June | December | December | June |

Read across the row and you have the answer for any date up to the 28th. The only work left is the handful of days that do not exist in the target month.

2. The tricky dates: the 29th, 30th and 31st

When the target month is shorter, move back to its last day. This is also what Excel's `=EDATE(A1,6)` does.

• March 31: September 30
• May 31: November 30
• August 29, 30 or 31: February 28, or February 29 in a leap year
• October 31: April 30
• December 31: June 30

▲ Short months pull the half birthday back to the last day

Why the last day and not the first day of the next month?

The point of a half birthday is "six months old."

October 1 would already be six months and one day for someone born March 31. Clamping to the end of the month keeps the person inside the sixth month.

Some families prefer a different convention for August 30 and 31, such as always using March 1. That is a family choice, not a calendar rule.

If you pick it, pick it once and use it every year so nobody's half birthday drifts.

And the leap-day birthday works the other way. February 29 plus six months is August 29 every year, so a leap-day baby gets a real half birthday more often than a real birthday.

3. Months or days? Why two calculators disagree

Some calculators count 182 or 183 days instead of six calendar months. A year has 365 days, so half of it is 182.5, and each tool rounds its own way. Months and days only line up by luck.

| Birthday | Six calendar months | Plus 182 days | Plus 183 days | |---|---|---|---| | January 27, 2026 | July 27 | July 28 | July 29 | | March 31, 2026 | September 30 | September 29 | September 30 | | July 4, 2026 | January 4 | January 2 | January 3 | | November 11, 2026 | May 11 | May 12 | May 13 | | December 31, 2026 | June 30 | July 1 | July 2 |

The day method drifts because the months it crosses are not the same length. February to July is short on days, so the count runs past the matching date.

July to January crosses two 31-day months in a row, so the count lands early.

Most people mean six calendar months, so that is the method used here.

If a site gives you July 28 for a January 27 birthday, it is counting days, not months, and neither answer is wrong. They are answering different questions.

4. Working it out in a spreadsheet

If you are planning half birthdays for a whole class or team, a spreadsheet is faster than doing them one by one.

• Excel and Google Sheets: put the birth date in A1 and use `=EDATE(A1,6)`. Format the cell as a date. Both programs clamp March 31 to September 30 the same way as the rule above
• This year's half birthday: `=EDATE(DATE(YEAR(TODAY()),MONTH(A1),DAY(A1)),6)` gives the half birthday that follows this year's birthday. If it is already past, add 12 to get next year's
• The weekday: `=TEXT(EDATE(A1,6),"dddd")` shows Monday, Tuesday and so on

One catch: for a February 29 birthday, the `DATE(YEAR(TODAY()),2,29)` part rolls over to March 1 in non-leap years. Type August 29 by hand for leap-day birthdays and skip the formula.

5. Half birthdays for summer birthday kids

Many schools let children with July and August birthdays celebrate a half birthday during the school year, since their real day falls in the holidays.

A child born August 31 celebrates on February 28 in 2027, which is a Sunday.

If the class party has to be on a school day, Friday February 26 is the nearest one. In 2028 the half birthday is February 29, a Tuesday.

A few practical points come up every year:

• Check the school calendar first. Mid-February often has a long weekend, so the nearest school day may be earlier than you think
• Agree on the convention with the teacher. If one family uses February 28 and another uses March 1 for an August 31 child, write down which one the class uses
• Use the half age, not just the date. Kids like hearing "seven and a half" more than "six months to go"

6. Checking the exact age on the day

The age calculator on ZAGO.TO shows today's exact age in years, months and days.

For a birthday on the 1st to the 28th, it reads six months and zero days on the half birthday itself.

For end-of-month birthdays it can read five months and 30 days instead, because it borrows the length of the month before, so go by the rule above.

It does not print a future half birthday date for you.

Use the table in section 1 or the `EDATE` formula for that, and use the calculator on the day to confirm the half age.

The method behind exact age is in age calculator: why exact age in years, months and days.

One-line answer

Add six months and keep the day. If that day does not exist, use the last day of the month.

Keep the family's birthdays in the ZAGO.TO age calculator so the exact ages are one tap away.

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