"It is 5:45."
Im not a japanese, but japanese doesnt seems to be very different from chinese. In chinese, we usually write 四十五分 for forty five minutes, but 四五分 is also correct because it meant 45 minutes too. The 四十五分 is kinda formal in daily conversation (in chinese), and we in a conversation we would use 四五分， where the 十 is left out to make it more casual.
I am Japanese and agree with Ryuko04. Old Chinese and Japanese didn't accept decimal number system. But they now accept it for convenience. The biggest issue for that is how to express zero. At least in Japanese, for example, 104 百四(traditional) or 一〇四(casual and decimal).