Mr. Sadler rightly emphasizes one aspect of time: that is to say time is one coordinate of a space-time set of coordinates that locate an event. The other aspect is that of time interval and it is this that the S.I. second defines. Thus if S.I. units are used the time interval between two events is measured in S.I. seconds; the orbital period of the Earth on its axis (referred to a specified frame of reference) is a certain number of S.I. seconds and, in this context, unless this happens to be 86,400 it is surely incorrect to call this interval a day.Charles Cotter says my note is controversial. I do not think this is so. The controversy lay in the decision to adopt S.I. units. My purpose was to indicate the implications of this decision. The S.I. unit of plane angle is the radian and hence the argument for the retention of a unit of distance related to the minute of arc is not relevant. The problem involved in adapting rhumb-line sailing to S.I. units is not difficult and has already been dealt with with great clarity. The advantages of a single universal coherent system cannot be over-emphasized and it is to be hoped that difficulties will not be prolonged by isolated areas of resistance.