Could you be
You change me once, You'll find me on your phone or next to it.
If you change the time once, you might find the "new" time on your phone or beside it, depending on where your phone is.
You change me twice, You'll find me on the sea or behind it.
If you change the time schedule on your phone (once) and then on your calendar (twice), you might be seeing the sun over the sea or over the horizon (behind it), depending on what your schedule is.
You don't change me, because you trust me.
We trust that a minute is 60 seconds; an hour is 60 minutes; and that every four years we have a leap year.
You can change me, because I trust you.
We invented the unit of time. Time is all around us, but humans invented the duration of a second, and its definition. It is, on some level, our own creation, and it has no choice but to trust us. We are all beings of time; our lives move forward in time.
They said, those two things make me exist.
Yes, and that is exactly what I said, too. It is because of us that we can measure time and understand its existence.
My roommate's short riddle.
It is short; it takes a short period of time to read it.
rm: Hi, good morning! I have a nice riddle for you.
Morning is a point in time.
me: seriously? Is this not too early?
Describes what point in time, morning is.
rm: c'mon, just guess who am I :)
c'mon, meaning "don't waste your time and get on with the riddle."