Hello, welcome to TPM!! I hope that you enjoy your stay and have fun posting at the forums!! ^_^
Anyways, unfortunately, there is no way to change your time in Pokémon Crystal. In Gold and Silver, there was a certain thing that you could do so that you could change the day and time on your game, it went like this...
After you turn your game on, and the title screen with Lugia (Silver version) or Ho-oh (Gold version) appears, on your Gameboy, press select, down, and B. Now, answer Yes to the question. It will then ask you to enter your password.
Your Password depends on your Trainer's name, ID number, and the amount of money that your trainer is currently holding. Note that only the money that you are holding counts, so don't figure in money that your mom is holding for you. Now, go to this page, http://filb.yagb.de/pwf/ and just enter your Trainer's name, (NOTE: Make sure that you enter your name exactly as you have it entered in your game, becuase your name is case-sensitive when it is being used to calculate this password!!) ID number, and the amount of money currently held into the boxes, then click the gray button that says "Get Password," and the password that you have to enter on your game will be displayed for you.
Now, the only problem, is that when Pokémon Crystal was released, Nintendo made sure to take out the whole time changing trick...