Birthdays are celebrated midnight to midnight on the date written on someone’s birth certificate.

If the time zones change one year – double British Summer Time, or GMT – the celebration still happens from midnight to midnight, local time.

When Amanda was born, her birth was defined in local time, where she was born. A date was written down on her birth certificate using that local time. If Amanda flies around the world, she always celebrates her birthday in local time, midnight to midnight of the anniversary date. When speaking of ages and of birthdays, we always, by tradition, speak in terms of the local time where we are now, and the local time where we were when we were born, and we calculate differences in terms of the local time numbers on the clocks that were on the wall then.

This is just a convention. Changing the local time when local politics changes the daylight savings adjustment, or one moves between time zones, does not really change one’s age measured in seconds since birth. It’s quite a strongly enforced convention. “People under 18 may not be sold alcohol,” the law might say. “Show us your ID,” the shopkeeper says. Someone born at 23:59 on June 1 in Australia could be about 36 hours younger than someone born at 00:00 on June 1 in the UK, but both would be allowed to buy alcohol in the UK at exactly the same time – 00:00 on June 1, 18 years after they were born.

So, birthdates conventionally relate to what’s going to be written on the birth certificate, a local date, in local time, stating the date (but not time) of birth. Age is computed using that date, those same local numbers, even if you move time zone. That’s how laws are written and interpreted, at least in the UK, and I suspect, everywhere.

Because of this, the birthdate calulator is able (thank goodness!) to ignore timezones completely. Dates and times are always assumed to be stated in the local timezone. Even if someone is born in one timezone and moves to another.

It would be possible to write a law saying, “People are first allowed to buy alcohol 568021248 seconds after they were born”.  They couldn’t buy alcohol a bit earlier by changing time zones. But, we don’t write laws like that. We write laws that refer to the number written on the birth certificate, even if those numbers were in some other time zone.