it is only for the DS
Yes, you can. Pokemon HeartGold uses the time from your DS settings. So, change the time on the DS and the time in Pokemon HeartGold will change with it.
Only on the Nintendo DS, DS lite and DSi
yes,
Yes, a Japanese version of Pokémon HeartGold will work on a DS from Australia. The Nintendo DS can play all games regardless of what country they originate from.
You need to mate the Pokemon with a Ditto.
no, but expected in 2010, heartgold & soulsilver will be released for the DS
No, HeartGold and SoulSilver are also for the DS and DSi considering it's the same game slot, but coming to your question, yes, it IS for the DS Lite so it will be playable if that's what you mean.
HeartGold and Soulsilver
When it is morning on the clock on the ds.
HeartGold and SoulSilver are DS remakes of Gameboy versions Gold and Silver.
You can bring you ds with heartgold in it to a local gamestop, and receive it through wifi