The invention relates to determining a quantity to be measured from a communication system, such as a transmission delay or the phase difference of clock times. Measurement messages are transmitted (501, 502) between the two areas of the communication system in both transmission directions. Values of the time difference are calculated (503) for the measurement messages transmitted in at least one of the transmission directions, each of which values is the difference between the instant of reception measured at the reception and the instant of transmission measured at the transmission of the measurement message. The values of the time difference are used to calculate (504) an estimate of the distribution of the time difference, on the basis of which an estimate of the minimum value of the time difference is calculated (504).