Although we say a year is 365 days long, that’s not strictly true: it’s actually a few hours longer. Until the middle ages, the precise length was thought to be 365 days and 6 hours; by then, more precise measures established it at 365 days 5 hours and 49 minutes. A slight difference, but one that over the course of centuries threw off the date-keeping by whole days. This was the motivation for a calendar revision and the introduction by Pope Gregory XIII of a new calendar – but it would take centuries more to spread beyond the Catholic countries.

On this day, January 12, in 1701, after several centuries of sticking to their prior form date-keeping, the Netherlands’ Protestant rulers finally decided to adopt the Gregorian calendar. Their reluctance was due to the Protestant Reformation – the new protestant movement was opposed to everything papal-decreed.

The Netherlands and Switzerland were still among the first to accept the Gregorian calendar. Russia was the last. When Alaska was bought from Russia in 1867, the resulting switch from Julian to Gregorian calendars skipped 12 days. The day after Friday, October 6th was Friday, October 18th.