According to the present invention there is provided in a first aspect a system (10) operable to determine a future point of time, at which the alcohol concentration in blood of a user is less than a threshold value. The system (10) comprises an alcohol sensor means (12) operable to detect a current alcohol concentration in blood of the user, and a portable apparatus (14). The alcohol sensor means (12) and the portable apparatus (14) comprises each a wireless communication means (I612; I614). The wireless communication means (I612) in the alcohol sensor means (12) is operable to wireless transmit the current alcohol concentration to the wireless communication means (I614) in the portable apparatus (14). The portable apparatus (14) comprises also a to the wireless communication means (I614) connected control means (18), a clock means (20), and a memory means (22), each connected to the control means (18). The control means (18) is operable to store the current alcohol concentration and a current time stamp from the clock means (20) in the memory means (22). The portable apparatus (14) comprises also a to the control means (18) connected calculation means (24) operable to calculate the future point of time in dependence of a known alcohol burn rate of the user stored in the memory means (22), or an alcohol burn rate calculated by the calculation means (24) in dependence of characteristics of the user which have an influence on the alcohol burn rate.