When someone says, “I’m 60 years old,” he means, “I have had my 60th birthday and have not yet had my 61st birthday.” Traditionally, someone born at any time from 00:00 (midnight) on june 1 to 23:59:59 (just before the next midnight) on June 1 has a birthday on June 1 every year. At least in the UK, by tradition, Independent of what time of day he was born, he will celebrate his birthday starting at 00:00 on each June 1, and will end his birthday at 23:59:59 on each June 1. 

Let’s set aside the effect of time zones on birthdays for another post. And let’s set aside the effect of changes of calendar, like the Julian-to-Gregorian change in 1752 in the UK, for another post too.

If the 60-year-old is speaking on a particular date , say yyyy/mm/dd, his 60th birthday could have been celebrated, at the latest, on yyyy/mm/dd. (So he’d be speaking on his 60th birthday). It could have been celebrated at the earliest on the day after (yyyy-1)/mm/dd. So his birthdate, the date written on his birth certificate, could have been anything from the day after (yyyy-61)/mm/dd up to (yyyy-60)/mm/dd.

“The day after” is just read off the calendar in the normal way. If (yyyy-1)/mm/dd is 1953/Dec/31, the day after is 1954/Jan/1. “The day after” 1953/Feb/28 was 1953/Mar/1. “the day after” 1952/Feb/28 was 1952/Feb/29, because 1952 was a leap year.

If you use the birthdate calculator to work out birthdate from someone’s stated age on a given day, and only fill in the ‘years’ field of the age, it’s understood that  you mean, they’ve celebrated that many birthdays, but you’re not sure when their birthday actually is. The birthdate calculator computes an earliest birthdate and a latest birthdate that could possibly make sense, according to the convention described above.

The birthdate tool works out when someone might have been born, given their age.

That doesn’t sound very hard, but turns out to be very tricky. If you find a bug, please leave a post here telling me what dates have gone wrong, and I’ll be very grateful.

Sometime in in March 2004, I saw the new baby. “Goodness, she’s so beautiful! How old is she?”, I said.

“She’s two months old”, came the reply.

OK, when was the baby born?

I know I asked the question in March 2004. So, on some day between 1 Mar 2004 and 31 Mar 2004, the baby was 2 months old.

If I had asked the question on 31 Mar 2004, the baby must have been born on or before 31 Jan 2004. If she’d been born on 1 Feb 2004, she’d not be 2 months old on 31 Mar. She’d be “Very nearly 2 months”, or something like that. So the latest possible birthdate for her is 31 Jan 2004.

If I had asked the question on 1 Mar 2004, the earliest she could have been born is 2 Dec 2003. If she’d been born on 1 Dec 2003, she’d have been 3 months old, not 2, on 1 Mar 2004.

So, there are two uncertainties, a vague age – “two months”, not “two months and 4 days” – and a vague date we asked the question, sometime in March. These two uncertainties combine to produce a range of possible birthdates, from earliest to latest. So the program produces two dates, an earliest possible birthdate, and a latest possible birthdate.

All this is of more than academic interest. I researched my family tree some while back, and was forever adding up ages and dates. I wrote a not-very-good spreadsheet to help. This web tool is intended to replace my spreadsheet, to be a more precise and general tool, and to be useful to others.

Much more on this later! Meanwhile, try out the tool at http://www.eversholt.org.uk/birthdate