# Birthdays

Today (Sunday 28th July) is my birthday, which naturally had me pondering birthdays in general. In July I have my birthday, my son’s birthday is a few days earlier and my granddaughter’s is next month. It turns out that a friend of a Facebook friend also has her birthday today, on my birthday. Jim Davis, the creator of Garfield was also born on my birthday. And finally, George, the latest addition to the royal family and third heir to the British throne was also born in July, the same month as me and my son.

The maths of birthdays is interesting. If you have 23 randomly selected people then the probability that at least two of them share a birthday is a shade greater than 50% (50.7297%). If you have 53 people the probability goes above 99%. This is known as the Birthday Problem or the Birthday Paradox, though it is not really a paradox, I believe. There are a number of simplification used in calculating the above. For instance, it assumes that all birth dates are equally probable, but they are not, and it also ignores leap days. Also mothers can sometimes, within bounds, select the day that their baby is born, especially for at risk babies and this potentially could cause a skew in the probabilities.

Some people have two birthdays. Well, the Queen has a real birthday and an official one, so that celebrations of her birthday would not fall too early in the year, but later, when the weather would hopefully be better. Unfortunately that means that in the Southern Hemisphere her birthday falls in the depths of winter!

Our years these days are defined in terms of “CE” or “Common Era” and “BCE” or “Before Common Era”. Older people can remember when it was “Before Christian Era” or even “BC” for “Before Christ” and “AD” for “Anno Domini” or “Year of Our Lord”.

I’m not going to argue whether or not Jesus really existed and whether or not he was divine, but if we assume for a moment that he was born, there is a lot of discussion on what year it was that he was born. Using the gospels and other historical information as a guide, many people believe that he was born 4 to 6 years before start of the Common Era. Or using the terminology, he was born up 6 years “Before Christ”! Humorous, I suppose.

Most people view history as continuous and the dates as fixed and well known. That’s not the case of course – the calendar has been revised several times, and  even countries which are Christian may have different calendars. Other religions naturally don’t relate their calendars to the birth of Christ. I believe that some even count backwards.

Calendars have grown out of necessity. Tax collectors in particular love calendars. Calendars are used to keep track of one’s age. Before calendars were widespread years were kept track of by relating births and deaths to important events, like the installation of a particular ruler. For instance, the gospel writer Luke relates Jesus’s birth to a census taken at the time:

<code>In those days Caesar Augustus issued a decree that a census should be taken of the entire Roman world. 2 (This was the first census that took place while[a] Quirinius was governor of Syria.) 3 And everyone went to their own town to register. Luke 2 1-3</code>

This shows the way that all dates were reckoned, relative to fairly recent events. The possibilities for error are obvious. Even if the events are written down, going more than a few years into the past involves research and calculation. Such calculations lead to such absurdities as Bishop Ussher’s calculation of the age of the earth as around 6,000 years. Even the dates of events early in the Common Era  can be dubious. This seems strange to citizens of the modern world, who can measure time to the accuracy of the vibration of an atom, and can accurately date events for at least a hundred or more years into the past.

One last comment – people who were born in the same year as the Queen, but born after her actual birthday and before her official birthday can claim to be both older and younger than the Queen.