1. A method of guiding an aircraft along a flight trajectory to comply with a time constraint for arriving at a required arrival time at a waypoint of said flight trajectory, comprising the steps of: A/ determining speed setpoints according to a speed profile that complies with the required arrival time at said waypoint based on estimated values of parameters relating to the flight of the aircraft, said speed setpoints being further determined by taking account of an upper limit for a maximum speed at which the aircraft flies along said flight trajectory (TV); and B/ applying the determined speed setpoints to the aircraft during guidance of the aircraft along said flight trajectory, wherein, in the course of the flight, in step A/: a) a current stretch is determined which depends on a current position of the aircraft and which corresponds to a stretch of the flight trajectory, comprising at least one part of the flight trajectory ahead of the current position of the aircraft, said current stretch being directly followed by a distant stretch which is defined along said flight trajectory at least up to said waypoint, wherein, in the course of the flight, in step A/, to implement predictions of speed setpoints, the following steps are further carried out: b) determining a first speed margin for the maximum speed of the aircraft along said distant stretch and an auxiliary upper limit, which is lower than said upper limit for said distant stretch, with the aid of the first speed margin; and c) determining the following: speed setpoints for said current stretch, by taking account of said upper limit, said speed setpoints relating to a guidance prediction for the guidance of the aircraft in step B/; and speed setpoints for said distant stretch, by taking account of said auxiliary upper limit to correct errors in the estimated values of parameters relating to the flight of the aircraft, wherein: in step b), a second speed margin is determined for the minimum speed of the aircraft along said distant stretch and an auxiliary lower limit, which is higher than a lower limit for said distant stretch, and with the aid of the second speed margin, said lower limit representing a minimum speed at which the aircraft can fly along said flight trajectory; and in step c), the following are further determined: speed setpoints for said current stretch are determined by taking account of said lower limit, said speed setpoints relating to the guidance prediction for the guidance of the aircraft in step B/; and speed setpoints for said distant stretch are determined by taking account of said auxiliary lower limit to correct errors in the estimated values of parameters relating to the flight of the aircraft.