In my opinion, nowadays there is room to be lenient when women wear underwear that is close to the body. It is almost impossible for a woman to have bled without noticing. If she had bled earlier, either she would have noticed it during the hefsek taharah or she would have seen that blood had come out.
All the exacting concern about the precise time of sunset really belonged to earlier times, when a woman wore many layers and it was possible that a drop of blood came out without her noticing.
Of course, if around sunset she spent a long time without underwear, she cannot rely on this leniency.

Thanks for the reference!
I saw that the responsum there deals with an examination that was definitely after sunset, and various opinions are brought there.
What happens in a case of doubt whether it was after sunset? In such a case, can one be lenient even according to those who prohibit?

One more thing: is it possible to know the exact sunset time in the area where I live with the resolution of a street or neighborhood?

Yes, because it is a case of doubt. And blessing will come upon one who is stringent.
I don’t know. Just look for tables online.