Present perfect with a past temporal expression

Question

I know the present perfect tense may not be used with temporal expressions which define a timeframe which does not include the present. For example:

WRONG: I've visited him on Monday.

However, when reading The Cambridge Encyclopedia of the English Language, this is what I have found:

The final section looks at what has happened to the English language in the 20th century, and in particular at its increasing presence worldwide.

I understand that grammar rules have their exceptions, but I can't wrap my head around why David Crystal used the present perfect instead of the past simple. It can't be a mistake, can it?

Answer 1

David Crystal wrote this sentence last century. Then the period of the English language changes was current and he could use only Present Perfect forms for the tendency.

Answer 2

I quote: "the [so-called] Present Perfect tense may not be used with temporal expressions." This is but a rule of thumb, as are most 'grammar rules'. Some Linguists will argue that there is such a thing as Perfect, even.

More importantly, in any case, 'have/has' has its place inasmuch as it serves a function [of communication], and tends to mean "this is the situation we HAVE"

Consider your example, "I've visited him on Monday" Although that example is rather unlikely, it is not impossible—and not "wrong" either: (Context and function rules, not grammar.)

Let's just say that it is the speaker's duty to visit that guy every Monday, and wants to report to his superior that NOW he HAS ACCOMPLISHED his task of visiting and acquiring the [necessary information] he needs for NOW. Then it's perfectly fine to say: "I've visited him on Monday" (regardless of whether he'll need to visit him the following week as well.) His visit was probably a success, though.