Lemme know if you don't understand any of the "lingo"...
Well, if you hit your date and time to the second that's good, but the trick is, you've gotta hit your "Delay" too. The delay is basically how many 60ths of a Second it takes you to get from Soft Reset to Pressing "Continue Game" on the Blue Screen. Yeah, 1/60 of a second T_T. Like I said, I've found that tapping to a good steady song helps a lot. My delays On HG are veeeeery consistently (at least 1/3 the time) 491 now.
If you started your game at the right date and time to the second, but were even 1 or 2 or 3 60ths off, it would give you the wrong "spread" for that Regigigas.
For example, with this Mewtwo that I just caught, I had to hit a much higher delay than normal (1061), so it took me 15 SRs just to hit that delay. I was hitting stuff like 1055/1067/1049/etc untill I just randomly landed on it and got mah Mootoo. xP