Patek Philippe patented the first perpetual calendar complication in wristwatch form in 1889 but did not market it until 1925. A highly complex mechanism, it displays and automatically adjusts the day, date and month – even on leap years – via two boxes at three and nine o’clock and a date display at six o’clock also featuring a moonphase. The sophisticated movement boasts a ‘mechanical memory’ of 1,461 days or four years. The reference 3970ER took over from the iconic reference 2499. Made throughout the 1990s, it was smaller in size and more compact. This is a beautifully constructed watch and we reckon a bit of a dark horse investment-wise.