I tried to calibrate, but I always keep on getting "No IVs matched", no matter how high I extended the delay and seconds range :(
Why?
So, here's what you do:
Enter your target delay and seconds into Eon Timer. It will give you a Minutes before target number. Save in front of Cobalion, then quit the game. Set your DS calendar and Clock to your target date. Set the time to the target time less the minutes before Eon Timer gives.
So, on my game I'm aiming for a target time of 28/5/2011 4:58:59, a delay of 27,952. (Yes, ridiculously high...) In Eon timer, I enter 59 into the target seconds box, and 27952 into the target delay. It gives a value of 8 minutes before. I then set my DS clock to 28/5/2011 at 4:50. I press A to set the time at exactly the same time as setting off Eon timer. (So, the first countdown begins at the same time I set the clock).
I turn the DS off (to get out of the time set menu) and start the game up. I get into the game (making sure to select no when it asks if I want the C GEar on, and then yes I'm sure). As soon as the first timer ends, I soft reset (L+R+Start+Select). I now get into the game again (again choosing no and yes I'm sure). Then you click the power button on the bottom screen. It asks if I'm sure I want to turn C Gear on. I wait at this screen.
As soon as the second timer finishes, I press yes with the stylus. Then I talk to Cobalion and catch it straight away. I then work out Cobalion's IVs, using metalkid's calculator and a few rare candies. I then Generate and Search IVs on the Seed to Time screen. It should find which delay I hit. Make sure you search a sufficiently wide range of delays before clicking Generate. Then I enter the actual delay I hit into Eon Timer, and Update. The Calibration number changes to reflect my results.
You can repeat this process a few times if you want to, but it shouldn't be that necessary.
Now you're ready to actually RNG. This time, instead of catching Cobalion straight away, first do the necessary frame advances, then catch it. Then check to see if you hit what you wanted to. (If not, you can manually search for the IVs you got near your seed in RNG Reporter's main window.)