As per H1B rules,
The Embassy or Consulate may process your H, L, O, P or Q visa application up to 90 days prior to the beginning of employment status as noted on your I-797.
You can apply for an H1B visa up to 90 days before the petition start date listed on the form I-797, Notice of Action.
Does this also apply to H1B DROPBOX appointments? If you joining date is say January 4, can you only take dropbox appointment dates after October 6?
What is September dates or August dropbox slots are available? Can you not take them?