Jewish Calendar (thing)
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|Because of the 19 year cycle, often the hebrew date coincides with the same english date every 19 years. Which means that for just about everyone, their 19th birthday will take place on the same day in both the english and hebrew calendars. This is not always the case. The Gregorian calendar and the Jewish calendar only synchronize every 80,000 years and change. (It takes about 230 years for the Jewish Calendar to fall a full day (and change) behind the Gregorian calendar.)<
P>As a Result some people may be unfortunate enough to be born during a year where the skew has become significant enough to warrant just enough shift that the date changes, and then their english and hebrew 19th birthdays won't coincide. This does not mean that we added an extra day, all the changes (ie. additions) that happen in the 19 year cycle happen in the 19 year cycle so those changes would repeat as well. Instead it means simply that the extra few (375 or so) seconds which accumulate each year which are not accounted for, just happen to be the last 375 seconds in a day, pushing us into the next day.
There is actually a table printed in most editions of the Tur, a major halachic work, in which there is an error regarding when the two calendars coincide. More details will be added when/if I can find the source in question.