Discussion of Meaning and Definition of UT

The new definition of UT1 adopted by the IAU is useful but for many reasons not quite satisfactory. It depends e. g., 1) on the approximate values of some astronomical constants, and is therefore subject to revision in the future. 2) Since it is used for the FK5-based astronomical reference system, its eventual usefulness for space techniques is questioned. 3) Although the new and old UT1 merge continuously at a chosen epoch, they do not form a homogeneous series of data, in other words, the old and the new UT1 are systematically different from each other. 4) Neither the new definition, nor the way to convert the old to the new one is based on simple concepts and these are thus likely to be misunderstood by the nonspecialist user. A conceptual definition of UT1 is suggested, in order to correct this situation and a formula to realize this conceptual definition is presented, which can be used unchanged for every technique and is easily understood by the nonspecialist community.