Taxes are prorated based on the month and day the taxes are due in a particular state.  The actual date the taxes are due varies from state to state and can be as early as May or as late as December.

The formula applied to prorate taxes is:

Annual tax bill: 365 days = one day’s tax (x) the number of days owed or refunded (driven by the closing date).

Assuming taxes are due on December 31 and the closing takes place on July 15, the seller will have paid no taxes for that year and is charged the prorated amount through July 15.  The buyer will therefore be charged prorated taxes beginning on July 16 and ending on December 31.