Careful not to break your timedate.cpl like I did

From what I gather, you have to take ownership of the file (not sure if that's the case with Restorator or not), I've heard a few people say to close Explorer.exe while editing it too. Open Restorator, open timedate.cpl in C:\Windows\System32, and drag the new .png files onto the ".PNG" folder. (I couldn't figure it out at first w drag and drop, has to go on top of the .png tab)

Save, reopen Explorer via Task Manager and restore ownership if you want.

I'm pretty sure that's how it's done, but you might want to poke around a bit first and make sure that's right. Moonique posted a video walkthrough in the time/date section.