If you are a single-member LLC or sole proprietor filing Schedule C for your business, the day you charge a purchase to your business or personal credit card is the same day you deduct the expense. Therefore, as a Schedule C taxpayer, you should think about using your credit card for last-minute purchases of office supplies and other business needs.
If you operate your business as a corporation, and if the corporation has a credit card in the corporate name, the same rule applies: The date of charge is the date of deduction for the corporation.
But if you are operating your business as a corporation and you are the personal owner of the credit card, the corporation must reimburse you if you want the corporation to realize the tax deduction, and that happens on the date of reimbursement. Therefore, submit your expense report and have your corporation make its reimbursements to you before midnight on December 31.