There is no harm in posting date at a later date. So long as the system allows you may post the entry. Only when the month is closed you may not pass back dated entries.
In SAP there are generally three dates. The relevant date which affects the books of account is the posting date while doc date and the entry date are just references. Entry date is the system date ie the day when you post the entry. Document date is the date on the face of the document.
It is recommended to post entries on a daily bases and avoid back dated entries.
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