RunAsDate is a tool by Nirsoft that loads any program with a user-specified time. It's useful for Gen 5 RNG and helps you hit your seed easily. This guide focuses only on RunAsDate.
Step 1: Setup RunAsDate
Launch RunAsDate.
Configure RunAsDate to match the image below.
Preview
You'll never need to change this again. This is the universal RunAsDate configuration for RNG (Gen 3, 4, or 5).
Select the program you want to fake the date/time using the Browse... button.
Set the date and time to one of the given times in PokeFinder to hit your seed.
Hit Run, and Desmume should launch.
Desmume will now run with the date and time you selected.
Step 2: Hitting the target seed
Load the Lua script.
Load your game.
Do any necessary keypresses to hit your target seed.
You can now continue as you would with basic RNG!
Troubleshooting
If you're not hitting the right seed on the first try with RunAsDate, check these things: