Validation of date , Posting key and Account

Hi,

I am getting a date it DDMMYY format, want to check if it is in the right format and then change to normal format (DD.MM.YYY). is there any FM for this?

the given posting keys and Accounts are Exist in SAP or not? how to check and

posting keys are stored in which table

Accounts stored in which table

Plz give me the table names of both posting keys and Accounts

i look forward to your reply

point will be rewarded

Thanks & Regards

SEK