you dont need to, autonumber creates a value with NO code.
Tho for your method, you would have to FORCE users to enter data from a form but NOT directly into the table. You'd need an 'entry' table, to verify the counter.
The form can get the date easy, but when the user clicks SAVE, it must ping the table for the current count, add 1, then build the key to be saved, then append the saved data.
Code:
private btnSave_Click()
dim vDate, vKey
dim iNum as integer
vDate =format( Date(),"yymmdd")
vKey= dMax("[Key],"table", vDate)
inum = mid(vKey, instrrev(vKey,"-")+1)
txtKey = vDate & & format(iNum + 1,"00")
docmd.openquery "qaPostData2Tbl"
end sub
again , lots of code that can break, when you only need autonum to do the job code free.