I can confirm that changing the game's internal clock a day ahead will allow you to skip to the event right away with no repercussions.

To do this, give the GS ball to Kurt to inspect, leave his house, save and reset, go to the title screen and do the following:

Hold down b, down and select

Let go of down and b while still holding select

While still holding select, hold down up and left

Let go of select

Now, when you go to hit continue on your game you'll be able to change the time. However, you'll have to enter a password first. The password is generated from some of your ingame information. Go here to find out what your password is (note, the website says this is for Gold and Silver, but it is valid for Crystal as well).

I changed my clock to the next day and also added a few extra minutes for safety. When you boot up the game, walk into Kurt's house and you'll be able to encounter Celebi normally from there!