Another thought. The above example seems more fitting for regular expected visits than for a once-off visit. Boss to salesman responsible for Dalian:
Have you visited Dalian in July? (Have you done your visit to Dalian for July?)
I have visited Dalian in July.

What I can glean from all the examples and discussion is that the "rule" that the Present Perfect cannot be used with "a SPECIFIC time in the past" is wrongly understood to apply across the board. I think it applies only in certain contexts, depending on the intended message, where the Simple Past should be the preferred or only correct form.

I shouldn't have opened this can of worms!!