The inventive timepiece comprises a watch movement, a calendar date mechanism (124) and a pair of hour and minute hands (10, 12) displaying a civil time and an additional minute hand (16) which displays a true minute and is driven by a true time display mechanism, wherein the day position of the true time minute hand (16) is determined by the position of a time equation cam (64) rotationally driven at one revolution per year by the calendar data mechanism (124). Said timepiece is characterised in that comprises a longitude adjusting mechanism (102) which makes it possible to put the true time minute hand (16) forward or back at a value of 1/-30 minutes by temporarily uncoupling it from the watch movement.