@SalixCatI'm also experiencing this issue for several months already. Here is what I deduced.
The traits of the other sims affect the duration of the relationship event. Say, I'm doing a friendly story and the Outgoing trait of my sim decreases the time of the event. Similarly the Outgoing trait of the other sim is supposed to decrease the time of the event (at least that is what was happening before). But I think some of the winter's updates removed this feature. The event card will still say the old duration, but the actual time would remove that boost from the trait of the other sim.
In the video below, you can see that the event card says 41 minutes for an event, but then it shows 48 minutes. The other sim has a relevant Alluring trait for this romantic story.
This happens with the sims of other players (the ones which have relevant traits) or with your own sims (like adult NPCs you haven't promoted who has Good Upbringing trait or retired sims).
Edit: I'm not sure why the video shows up really small. I will include screenshots below.
Here are the screenshots
@yani_tsm Thanks for the screenshots.
The way in which the information is shown on the cards was changed in the High Fashion update (September 2019).
Here's an example. On the left side you can see how it was before the High Fashion update. It showed same time (1 h 18 s) when Bella (adult w/out good upbringing) had a family event with a child who already had good upbringing. After the update, it showed 57 min. on the card and an effective duration of 1 hour. Those 3 minutes are due to the kid's GU trait, not Bella's as she doesn't have GU.
Tap or click on the image to enlarge:
I asked then for an explanation. The answer I had is that nothing's actually changed in terms of the actual event duration as the 18s are just rounded off.
