5:30pm   exactly 1 hour later would be 6:30pm

but we're looking 45 minutes later
so subtract 60 from 45

60-45= 15 minutes
now that we got the difference between 60 and 45 
we subtract the sum to the 1 hour later

what do I mean?
6:30 - 15 minutes = 6:15 ---->  6:15pm is exactly 45 minutes later