I didn't get the 7 on the 7th achievement for Halo 3?

[PokemonPlatinumFan]

On January 7th I started with 6 exp. and then I reached 7 exp. and didn't get the 7 on the 7th achievement. So I need to know what I did wrong and how can I try to get the achievement again. Please help me!
I know that you can only get this achievement on the 7th of the month! What I meant by How can I get this achievement again? was that how do I get the achievement if I already failed at getting it once!
I have a minor question: Do you have to get 7 exp for the World Ranking system or for Playlist Ranking system?

 

[Acid Downpour]

I don't think you'd be able to try and get that achievement again as you can only get it on that specific day, being the 7th of January. Sorry.

 

[thegamevault]

You have to enter a playlist with 7 exp on the 7th of the month. So once you reached 7 exp, you should've played another game in that playlist. Also, the exp is per playlist, not your overall total exp.