[trivial] fix datetime test

If `LocalTime()` == `UTC()`, we created a AA literal with two same keys: 
```
auto diffAA = [0 : Rebindable!(immutable TimeZone)(UTC(),  
                      0 : Rebindable!(immutable TimeZone)(LocalTime()), 
                      ot : Rebindable!(immutable TimeZone)(otherTZ)];
```
It was allowed in old AA implementation, however it isn't good.