Hi Heike,
Post a screen of your error so as we will have an idea regarding the error..Numbers and characters in a CHAR type infoobject is allowed as long the special characters were added in tcode RSKC..
Don't have an idea regarding the 2nd question if there exists a type which can do what you want..But you may try two (2) infoobjects: one (1) for TIME char and antoher one (1) for DATE char..
Regards,
Loed