This is strange, but also permanent, so only do this if you want your clock to remain set on a certain day. To do this, go to Vermillion City and talk to the Machop that is stomping on the ground. After you talk to him, open your Pack, go to Coin Case, and look at how many coins you have. Your game will either freeze or reset, so you must reset if it freezes. When you do, your game clock will be frozen in that exact time unless you use another code to change the time, which is possible