There is on average, 730 hours a calendar month. A. Lange & Söhne's Lange 31 has a power reserve that tops that length of time, clocking in a staggering 744 hours.
At first sight, the sleek, grey dial of the LANGE 31 in the 45.9-millimetre white-gold case does not reveal any sign of the power hidden inside its manually wound calibre.
Only the white 31 at the upper end of the large power-reserve indicator and the small inscription ("Monats-Werk") beneath the Lange outsize date modestly refer to the fact that when fully wound, the movement will run for an entire month.
It is a formidable technical challenge to endow a mechanical watch with a power reserve of 744 hours – and so is assuring superb rate accuracy from the first to the last day. With the launch of the LANGE 31 in 2007, introducing the calibre L034.1, it became reality.
To run the entire 31-day period, the energy is stored in two stacked mainspring barrels. By inserting and rotating a square key through an aperture in the sapphire-crystal caseback, the spring barrels gain the energy needed. With another complicated mechanism, the constant-force escapement, the power delivery is transmitted gently and uniformly into the movement over the entire month.