This is a common question. Typically, backdating counseling statements is unacceptable, especially if it puts the Soldier at a disadvantage. There are no Field Manuals or Regulations that specifically address this matter.
The DA Form 4856 is a written record of the counseling session and should be completed during or immediately after a counseling session. In my humble opinion the DA Form 4856 should reflect the actual date the form is signed. The contents of the counseling could reference a previous date. For example: if you were verbally counseled on 2 Nov for an offense, and this offense was later documented in a written counseling statement on 10 Nov. It would be appropriate to mention the date of the verbal counseling in the written statement and the Soldier should sign the written document with a date of 10 Nov.