Hi,
I don't know if this has already been done by someone, but yesterday I did some research in my Platinium save file.
EDIT : Now in HGSS.
Here is the results (in addition to the wiki) (all offsets are relative to the starting offset of a block) :
Pokémon Platinium
General block offsets :
Adventure started : 0x34
Unsigned 32-bit integer
In seconds since 1/1/2000 00:00:00
Pokémon League Champion date : 0x3C
Unsigned 32-bit integer
In seconds since 1/1/2000 00