I recently met my first example of someone born on February 29, Leap Day. I did realize earlier that such a birthday would be misleading in ordinary terms. The woman we met said that she had experienced her birthday something like 18 times. So, I figure in ordinary years, she is probably 18 x 4 years old.
My lifetime has been very much affected by computers and my first instruction in computer use was in graduate school. A small group of us labored for 3 years as explained here:
https://fearfunandfiloz.blogspot.com/2009/08/what-day-will-christmas-fall-on-this.html
The calendar we use is the Gregorian calendar, instituted by Pope Gregory XIII and his aides in 1582. The Pope had been advised that the Julian calendar had, over time, gotten out of whack and needed to be adjusted. In October, he declared a given day would be followed by another date eleven days into the future. There were riots as people declared they demanded their days back.
From an earlier post in this blog:
Researching possible challenges he might give us, we learned that in 1752, all British lands, including the American colonies, agreed to accept the calendar adjustments created much earlier under Pope Gregory XIII to correct the calendar. The Pope's astronomers had calculated that the calendar needed to omit 11 days for better adjustment and different countries adopted the correction at different times. The British lands had riots over the correction, where some workers feared the shortened month would result in less pay. It seems that the calendar riots resulted in full pay for that month's work.
I searched "how many different calendars are in current use". I knew it was a tricky question. I found
There are seven calendars in regular current use around the world. They are the Gregorian, the Chinese, the Hebrew, the Islamic, the Persian, the Ethiopian and the Balinese Pawukon. The Gregorian is used worldwide for business and legal reasons. The others are sometimes used for religious and sometimes social reasons. Modern Calendars - Calendopaedia
I also found
forty calendars
According to a recent estimate, there are about forty calendars used in the world today, particularly for determining religious dates. Most modern countries use the Gregorian calendar (see the Year) for their official activities.
A Variety Of Calendars | Calendars - Webexhibits
