This has been a timely reminder to us all.

[kadioguy]

This has been a timely reminder to us all.
https://www.oxfordlearnersdictionaries.com/definition/english/timely?q=timely

1. Why is the present perfect used? What does that mean?

Does it mean this? "This" (something) happened in the past, and from the view point of now, we see it as a timely reminder.

2. But how about this?

b. This is a timely reminder to us all. (My sentence)

I assume that (b) wouldn't make sense, because that happened in the past. Is that right? But I think that there could be some case where (b) can be used. What do you think? Could you give an example?

 

[Tarheel]

1. It happened in the past, but it's relevant now.
2. That seems unlikely.

 

[jutfrank]

I assume that (b) wouldn't make sense, because that happened in the past. Is that right? But I think that there could be some case where (b) can be used.

Right. We can't very well say why the speaker used present perfect in this case, as we don't have the context.