The American Taxpayer Relief Act of 2012, continues the undification of the gift and estate tax. By unifying the gift and estate tax, Congress has kept the two exemptions the same along with the generation-skipping exemption. In 2013, due to indexing, that exemption is $5,250,000.
The way the exemption works is that the unused portion of the exemption is available to the estate at the death of the decedent. For example, if the decedent dies in 2013 and made gifts in 2012 of $5,000,000 anticipating that the exemption would disappear, the decedent's estate has $250,000 of unused exemption to apply to the transfers to the decedent's heirs or beneficiaries.
The unused exemption is calculated by "clawing back" into the decedent's taxable estate the value of all the lifetime taxable gifts made by the decedent. The formula for calculating the tax liability is then applied to the gross taxable estate (which includes the clawed back gifts).
When are those taxes due? Nine months from the date of the decedent's death.