Born Upon This Day

Is it Isaac Newton's birthday or not?

He was born on December 25, 1642 - as dates were then reckoned in England.

Some years after his death, the calendar system was revised. By this new system he was retroactively born on January 4, 1643.

So I guess we can celebrate his birthday twice.

When calendars are re-reckoned
You get one birthday - and then a second.