Marvin is right about saying "It's a quarter to one." "Quarter" means 1/4 (one fourth) and 1/4 of 60 minutes is 15 minutes. There are other ways to say "12:45," we may say "It's fifteen to one," or simply "it's fifteen 'till," but the most popular way of all is to say "It's twelve forty five." Also, if it's 1:15, we say "It's one fifteen" or "It's a quarter past one."