I've tried changing the clock but all Mr Backlot says is that Marill is in his garden. Also if there's another way to change the Pokemon that appears put that in the answer as well
I’m pretty sure just changing the internal clock doesn’t work, since I’m certain that it doesn’t work with honey trees.
However the bulbapedia article for trophy garden mentioned something about soft reseting. I looked it up and found this guide.
Save before talking to Mr. Backlot and soft reset the game if he doesn’t mention your desired Pokemon. Read the previously mentioned guide for further details.