How is the Universe here?

I read over the posts that described how time is different for each oberserver; how a man on mercury would see a solar flare before we do on earth. I get that. What I don't get is how it relates to time. If the solar flare happend at 4:00 p.m. on the sun than it still happend a 4:00 p.m. for observers on earth, they just havn't observed it yet.If my friend, who livers 30 minutes away, leaves his house at 4:00 and arrives ay my place at 4:30, I wouldn't say he left his house at 4:30 and also arived at 4:30. I would know he left at 4:00 and it took him 30 minutes to get here. Isn't this the same thing as the sun example?Or am I being totally close minded? Can anyone elaborate more for me?

Yes, but how does one know if time actually slows down because of velocity/gravitational fields? What if only the clock is slowed down?How would you know either way?