Time is a well defined physical property that has nothing to do with the posistion of the sun.

We already have time localizations, time zones, daylight savings time, and leap days all handled by layers above UTC (actually, all in the timezone layer). Leap seconds should be part of that layer, not the UTC layer.

Once we get into relativistic effects, there is more complication to the lower layers, but we (mostly) don't need to deal with that.