Apostrophes with Contractions

Use apostrophes in contractions to show where letters are omitted. Consider these examples:

cannot (can't)
had not (hadn't)
it is (it's)
you are (you're)
have not (haven't)
who is (who's)
let us (let's)
we have (we've)
I am (I'm)
of the clock (o'clock)
summer of 1968 (summer of '68)

Be careful when using its or it's and whose or who's. Since these words are pronounced alike, they are often misused. Learn to proofread for them.

Possessive -----> Contraction
its -----> it's (it is or it has)
whose -----> who's (who is)

Which sentence is correct? Because the luggage has no identification tags, its impossible to figure out who's bags they are. I'm not exactly sure when we're arriving, but it's supposed to be some time after six o'clock in the evening.