The purpose of this article is twofold: firstly to give readers a general account of the new time-system that will be introduced on 1 January 1972, and secondly to give greater detail in respect of the corrections that a relatively few navigators may need to apply.

The great majority of users will not be (and need not be) aware of any change.

In his paper ‘Time Data and Navigation’ (page 13 of this Number) H. M. Smith has given an excellent background to the change, but that paper was written shortly before the final details were agreed; a little background material is, however, repeated here.