Quick answer
A half birthday is your birth date plus six months. Born on 17 May 1990, the half birthday falls on 17 November, a Saturday in 1990.
Enter your date of birth. The calculator returns the half-birthday date and its weekday.
Related tools: Birthday Calculator: Next birthday, days until it, and your current age. Baby Age Calculator: A baby's age in weeks, months and days. Age Difference Calculator: Time between two birth dates.
How to use the Half Birthday Calculator
- Pick date of birth.
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Date of birth | A calendar date | — |
Formula and method
Formula
half birthday = birth date + 6 months
IEEE-754 double precision; shown to the decimals indicated.
If the month is shorter, the last day of the month is used.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Date of birth = 1990-05-17.
| Result | Value |
|---|---|
| Half birthday | 1990-11-17 |
| Weekday | Saturday |
Example 2
Inputs: Date of birth = 2001-08-31.
| Result | Value |
|---|---|
| Half birthday | 2002-02-28 |
| Weekday | Thursday |
Example 3
Inputs: Date of birth = 1999-12-25.
| Result | Value |
|---|---|
| Half birthday | 2000-06-25 |
| Weekday | Sunday |
The first line is the headline result (half birthday). Below it the half Birthday Calculator lists weekday, which describe the same case from other angles. IEEE-754 double precision; shown to the decimals indicated.
Edge cases
Someone born on 31 August has no matching day six months later, because February has no 31st. Adding six months to 31 August lands on 28 February (29 in a leap year), the last valid day of that month, and the calculator shows that nearest valid date.
Why people care
Half birthdays are used for children's celebrations and for adults who like to mark the halfway point to the next birthday.
What a half birthday is
A half birthday is the date six months after a birth date: 17 May 1990 gives 17 November 1990, which fell on a Saturday. It is an informal custom, not an official date.
Birthdays on the 29th to the 31st are the awkward case, because the month six later may be shorter. Treat the date shown as a convention and choose the nearest day you prefer.
Half Birthday from different start dates
The same calculator run from 1 January of four consecutive years.
| Date of birth | Half birthday | Weekday |
|---|---|---|
| 2026-01-01 | 2026-07-01 | Wednesday |
| 2027-01-01 | 2027-07-01 | Thursday |
| 2028-01-01 | 2028-07-01 | Saturday |
| 2029-01-01 | 2029-07-01 | Sunday |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Date of birth: left empty shows "Enter date of birth." beside that field and no result.
Choosing between the Half Birthday Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Birthday Calculator](/lifestyle/birthday/) | date of birth and reference date | Next birthday |
| [Baby Age Calculator](/lifestyle/baby-age/) | date of birth and reference date | Age |
| [Age Difference Calculator](/lifestyle/age-difference-calculator/) | first birth date and second birth date | Age difference |
| [School Age Calculator](/lifestyle/school-age/) | date of birth and school cutoff date | Age on the cutoff date |
| [Birth Date from Age Calculator](/lifestyle/age-to-birthday/) | age in years and reference date | Possible birth dates |
Assumptions and limitations
- Day clamped in shorter months
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for date of birth to confirm this.
Frequently asked questions
Is a half birthday exactly 6 months?
Yes, six calendar months after the birth date.
Can I find mine for this year?
Use a recent year as the starting date.
Does it consider leap years?
Only for 29 February births.
Is a half birthday exactly six months later?
By custom yes, the same day of the month six months on, though it is informal and families choose their own rules.
Why does the weekday change?
Six months is not a whole number of weeks, so the half birthday lands on a different weekday each year.
What does the Half Birthday Calculator calculate?
The date six months after a birthday. It returns Half birthday and Weekday.
What formula does the Half Birthday Calculator use?
half birthday = birth date + 6 months. If the month is shorter, the last day of the month is used.
What do I need to enter in the half Birthday Calculator?
You enter Date of birth.
How is the result rounded or limited?
IEEE-754 double precision; shown to the decimals indicated. Day clamped in shorter months.
What is an example of a half Birthday result?
With date of birth 1990-05-17, the half birthday is 1990-11-17.
Where does the method come from?
The method follows: ISO 8601-1: Date and time, representations for information interchange (calendar dates, ordinal dates and week dates) (calendar-date arithmetic); a half-birthday is the date six months after a birthday, an informal convention.
Related calculators
- Birthday Calculator — Next birthday, days until it, and your current age.
- Baby Age Calculator — A baby's age in weeks, months and days.
- Age Difference Calculator — Time between two birth dates.
- School Age Calculator — Child's age on a school cutoff date.
- Birth Date from Age Calculator — Range of possible birth dates for someone of a given age.
- Weekday Calculator — Look up the weekday (Monday to Sunday) of any past or future date.
- Half-Life Calculator — Remaining amount, elapsed time or half-life from exponential decay.
- Graduation Year by Birthday Calculator — High school graduation year from a birth date and your school's entry rules.
Sources
- ISO 8601-1: Date and time, representations for information interchange (calendar dates, ordinal dates and week dates) (calendar-date arithmetic); a half-birthday is the date six months after a birthday, an informal convention
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.