I am not sure that I understand your below mail completely.
With regard to any new hire - new joinees leave starts from the date of his/her joining. So in the case of the new hire who joined on 18th August if the employee has taken any leave will be shown in September which will be reflected in the T& A sheet. Therefore, I would request you to discuss internally with your colleagues who had done similar cases in the past for new hires.
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